Martensitic stainless steel material

ABSTRACT

A martensitic stainless steel material contains, in mass %, C: 0.030% or less, Ni: 5.00 to 7.00%, Cr: 10.00 to 14.00%, and Cu: more than 1.00 to 3.50%. On two line segments LS of 1000 μm extending in a wall thickness direction with arbitrary two points as a center located at positions at a depth of 2 mm from the inner surface, respectively, a degree of Cr segregation ΔCr defined by Formula (1) described in the description, a degree of Mo segregation ΔMo defined by Formula (2) described in the description, and a degree of Cu segregation ΔCu defined by Formula (3) described in the description satisfy Formula (4):ΔCr+ΔMo+ΔCu≤A  (4)where, when the yield strength is 758 to less than 862 MPa, A in Formula (4) is 0.70, and when the yield strength is 862 MPa or more, A in Formula (4) is 0.50.

TECHNICAL FIELD

The present disclosure relates to a steel material, and moreparticularly relates to a martensitic stainless steel material that is aseamless steel pipe or a round steel bar.

BACKGROUND ART

In oil wells and gas wells (hereunder, oil wells and gas wells arecollectively referred to as “oil wells”), a steel material referred toas a downhole member is used that has been processed into apredetermined shape from a seamless steel pipe or a round steel bar. Oilwells are being made deeper in recent years, and consequently there is ademand to enhance the strength of steel materials to be used for oilwells. Specifically, steel materials for oil wells of 80 ksi grade(yield strength is 80 to less than 95 ksi, that is, 552 to less than 655MPa) and 95 ksi grade (yield strength is 95 to less than 110 ksi, thatis, 655 to less than 758 MPa) are being widely utilized. Furthermore,requests have also recently started to be made for steel materials foroil wells of 110 ksi grade (yield strength is 110 to less than 125 ksi,that is, 758 to less than 862 MPa).

In this connection, most deep wells are in sour environments thatcontain corrosive hydrogen sulfide. In the present description, the term“sour environment” means an acidified environment containing hydrogensulfide, or hydrogen sulfide and carbon dioxide. Steel materials to beused in such sour environments are required to have not only theaforementioned high strength, but also to have excellent sulfide stresscracking resistance (hereunder, referred to as “SSC resistance”).

The H₂S partial pressure in a sour environment differs depending on theregion. In sour environments (mild sour environments) in which the H₂Spartial pressure is 0.03 bar or less, martensitic stainless steelmaterials containing about 13% by mass of Cr that are typified by an APIL80 13Cr steel material (normal 13Cr steel material) and a Super 13Crsteel material in which the content of C is reduced are used. However,in a sour environment (enhanced mild sour environment) in which the H₂Spartial pressure is in the range of more than 0.03 to 0.10 bar or lessthat is higher than in a mild sour environment, SSC resistance that ishigher than in the aforementioned normal 13Cr steel material and Super13Cr steel material is required.

Steel materials having higher SSC resistance than the aforementionednormal 13Cr steel material and Super 13Cr steel material are proposed inJapanese Patent Application Publication No. 10-001755 (Patent Literature1), Japanese Translation of PCT International Application PublicationNo. 10-503809 (Patent Literature 2), and Japanese Patent ApplicationPublication No. 08-246107 (Patent Literature 3).

A martensitic stainless steel material according to Patent Literature 1has a chemical composition consisting of, in mass %, C: 0.005 to 0.05%,Si: 0.05 to 0.5%, Mn: 0.1 to 1.0%, P: 0.025% or less, S: 0.015% or less,Cr: 10 to 15%, Ni: 4.0 to 9.0%, Cu: 0.5 to 3%, Mo: 1.0 to 3%, Al: 0.005to 0.2%, and N: 0.005% to 0.1%, with the balance being Fe andunavoidable impurities, and satisfying 40C+34N+Ni+0.3Cu−1.1Cr−1.8Mo≥−10.The microstructure of the martensitic stainless steel material disclosedin this patent literature consists of a tempered martensite phase, amartensite phase, and a retained austenite phase. A total fraction ofthe tempered martensite phase and the martensite phase in themicrostructure is 60% or more to 80% or less, and the balance is theretained austenite phase.

A martensitic stainless steel according to Patent Literature 2 consistsof, in mass %, C: 0.005 to 0.05%, Si≤0.50%, Mn: 0.1 to 1.0%, P≤0.03%,S≤0.005%, Mo: 1.0 to 3.0%, Cu: 1.0 to 4.0%, Ni: 5 to 8%, and Al≤0.06%,with the balance being Fe and impurities. Further, the aforementionedchemical composition satisfies Cr+1.6Mo≥13, and40C+34N+Ni+0.3Cu−1.1Cr−1.8Mo≥−10.5. The microstructure of themartensitic stainless steel of this patent literature is a temperedmartensite structure.

The chemical composition of a martensitic stainless steel according toPatent Literature 3 consists of, in mass %, C: 0.005% to 0.05%, Si:0.05% to 0.5%, Mn: 0.1% to 1.0%, P: 0.025% or less, S: 0.015% or less,Cr: 12 to 15%, Ni: 4.5% to 9.0%, Cu: 1% to 3%, Mo: 2% to 3%, W: 0.1% to3%, Al: 0.005 to 0.2%, and N: 0.005% to 0.1%, with the balance being Feand unavoidable impurities. Further, the aforementioned chemicalcomposition satisfies 40C+34N+Ni+0.3Cu+Co−1.1Cr−1.8Mo−0.9W≥−10.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Publication No.    10-001755-   Patent Literature 2: Japanese Translation of PCT International    Application Publication No. 10-503809-   Patent Literature 3: Japanese Patent Application Publication No.    08-246107

SUMMARY OF INVENTION Technical Problem

In the martensitic stainless steel materials for oil wells proposed inPatent Literature 1 to Patent Literature 3, adequate SSC resistance in asour environment is obtained by adjusting the contents of the respectiveelements in the chemical composition based on a parameter formula.However, adequate SSC resistance in a sour environment together withhigh strength may be obtained by another means that is different fromthe means proposed in Patent Literature 1 to Patent Literature 3.

An objective of the present disclosure is to provide a martensiticstainless steel material that has high strength and is excellent in SSCresistance.

Solution to Problem

A martensitic stainless steel material according to the presentdisclosure is as follows.

A martensitic stainless steel material that is a seamless steel pipe ora round steel bar, having a chemical composition consisting of, in mass%:

-   -   C: 0.030% or less,    -   Si: 1.00% or less,    -   Mn: 1.00% or less,    -   P: 0.030% or less,    -   S: 0.0050% or less,    -   Ni: 5.00 to 7.00%,    -   Cr: 10.00 to 14.00%,    -   Mo: 1.50 to 3.00%,    -   Al: 0.005 to 0.050%,    -   V: 0.01 to 0.30%,    -   N: 0.0030 to 0.0500%,    -   Ti: 0.020 to 0.150%,    -   Cu: more than 1.00 to 3.50%,    -   Co: 0.50% or less,    -   B: 0 to 0.0050%,    -   Ca: 0 to 0.0050%,    -   Mg: 0 to 0.0050%,    -   rare earth metal (REM): 0 to 0.0050%,    -   Nb: 0 to 0.15%,    -   W: 0 to 0.20%, and    -   the balance: Fe and impurities,    -   wherein:    -   a yield strength is 758 MPa or more;    -   in a case where the martensitic stainless steel material is the        seamless steel pipe,    -   when, in a cross section including a rolling direction and a        wall thickness direction of the seamless steel pipe, an        arbitrary two points at positions at a depth of 2 mm from an        inner surface are defined as two center points P1, and two line        segments of 1000 μm extending in the wall thickness direction        with each center point P1 as a center are defined as two line        segments LS, energy dispersive X-ray spectroscopy is performed        at measurement positions at a pitch of 1 μm on each line segment        LS, and a Cr concentration, a Mo concentration, and a Cu        concentration at each measurement position are determined;    -   in a case where the martensitic stainless steel material is the        round steel bar,    -   when, in a cross section including a rolling direction and a        radial direction of the round steel bar, an arbitrary two points        on a central axis of the round steel bar are defined as two        center points P1, and two line segments of 1000 μm extending in        the radial direction with each center point P1 as a center are        defined as two line segments LS, energy dispersive X-ray        spectroscopy is performed at measurement positions at a pitch of        1 μm on each line segment LS, and a Cr concentration, a Mo        concentration, and a Cu concentration at each measurement        position are determined; and    -   when:    -   an average value of all of the Cr concentrations determined at        all of the measurement positions on the two line segments LS is        defined as [Cr]_(ave),    -   a sample standard deviation of all of the Cr concentrations        determined at all of the measurement positions on the two line        segments LS is defined as σ_(Cr),    -   among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, an average        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) is defined as [Cr*]_(ave),    -   among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) is defined as [Cr*]_(max),    -   among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) is defined as [Cr*]_(min),    -   an average value of all of the Mo concentrations determined at        all of the measurement positions on the two line segments LS is        defined as [Mo]_(ave),    -   a sample standard deviation of all of the Mo concentrations        determined at all of the measurement positions on the two line        segments LS is defined as σ_(Mo),    -   among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, an average        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) is defined as [Mo*]_(ave),    -   among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) is defined as [Mo*]_(max),    -   among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) is defined as [Mo*]_(min),    -   an average value of all of the Cu concentrations determined at        all of the measurement positions on the two line segments LS is        defined as [Cu]_(ave),    -   a sample standard deviation of all of the Cu concentrations        determined at all of the measurement positions on the two line        segments LS is defined as σ_(Cu),    -   among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, an average        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) is defined as [Cu*]_(ave),    -   among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) is defined as [Cu*]_(max), and    -   among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) is defined as [Cu*]_(min),    -   a degree of Cr segregation ΔCr defined by Formula (1), a degree        of Mo segregation ΔMo defined by Formula (2), and a degree of Cu        segregation ΔCu defined by Formula (3) satisfy Formula (4):

ΔCr=([Cr*]_(max)−[Cr*]_(min))/[Cr*]_(ave)  (1)

ΔMo=([Mo*]_(max)−[Mo*]_(min))/[Mo*]_(ave)  (2)

ΔCu=([Cu*]_(max)−[Cu*]_(min))/[Cu*]_(ave)  (3)

ΔCr+ΔMo+ΔCu≤A  (4)

where, in a case where the yield strength is 758 to less than 862 MPa, Ain Formula (4) is 0.70, and in a case where the yield strength is 862MPa or more, A in Formula (4) is 0.50.

Advantageous Effects of Invention

The martensitic stainless steel material according to the presentdisclosure has a high strength that is a yield strength of 110 ksi ormore (758 MPa or more), and is excellent in SSC resistance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional diagram along a direction perpendicular to alongitudinal direction of a starting material of a martensitic stainlesssteel material of a present embodiment.

FIG. 2 is a cross-sectional diagram along a direction perpendicular to arolling direction of a seamless steel pipe.

FIG. 3 is a cross-sectional diagram including the rolling direction anda wall thickness direction of the seamless steel pipe.

FIG. 4 is an enlarged view of a vicinity of center points P1 in FIG. 3 .

FIG. 5 is a multiple view drawing including a cross-sectional diagramalong a direction perpendicular to a rolling direction of a round steelbar, and a cross-sectional diagram along a direction parallel to therolling direction of the round steel bar.

FIG. 6 is a schematic diagram of a heating furnace that is utilized in aprocess for producing the martensitic stainless steel material of thepresent embodiment.

FIG. 7A is a view illustrating a relation between an FA value that is aheating condition and a total degree of segregation ΔF of themartensitic stainless steel material of the present embodiment in a casewhere a yield strength of the steel material is made 110 ksi grade (758to less than 862 MPa).

FIG. 7B is a view illustrating a relation between an FA value that is aheating condition and a total degree of segregation ΔF of themartensitic stainless steel material of the present embodiment in a casewhere the yield strength of the steel material is made 125 ksi or more(862 MPa or more).

DESCRIPTION OF EMBODIMENTS

The present inventors conducted studies regarding a steel material inwhich a yield strength of 110 ksi or more (758 MPa or more) andexcellent SSC resistance in a sour environment can be compatiblyobtained.

First, the present inventors conducted studies regarding a steelmaterial in which a yield strength of 110 ksi or more and excellent SSCresistance can be compatibly obtained, from the viewpoint of the designof the chemical composition. As a result, the present inventorsconsidered that if a steel material consists of, in mass %, C: 0.030% orless, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S:0.0050% or less, Ni: 5.00 to 7.00%, Cr: 10.00 to 14.00%, Mo: 1.50 to3.00%, Al: 0.005 to 0.050%, V: 0.01 to 0.30%, N: 0.0030 to 0.0500%, Ti:0.020 to 0.150%, Cu: more than 1.00 to 3.50%, Co: 0.50% or less, B: 0 to0.0050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, rare earth metal (REM): 0to 0.0050%, Nb: 0 to 0.15%, and W: 0 to 0.20%, with the balance being Feand impurities, there is a possibility that a yield strength of 110 ksior more and excellent SSC resistance in a sour environment can becompatibly obtained.

Therefore, the present inventors produced a steel material having theaforementioned chemical composition by a well-known method, andevaluated the yield strength and SSC resistance in a sour environment.As a result, the present inventors found that, simply by adjusting thecontents of the elements in the chemical composition, a yield strengthof 110 ksi or more and excellent SSC resistance in a sour environmentare not necessarily adequately obtained compatibly in some cases.Therefore, the present inventors conducted various studies toinvestigate the reason why, in some cases, a yield strength of 110 ksior more and excellent SSC resistance in a sour environment cannot becompatibly obtained in a steel material having the aforementionedchemical composition. As a result, the present inventors obtained thefollowing findings.

In the chemical composition described above, the SSC resistance of thesteel material in a sour environment is improved by making the contentof Cr 10.00 to 14.00%, the content of Mo 1.50 to 3.00%, and the contentof Cu more than 1.00 to 3.50%, and setting the contents of the otherelements to be within the aforementioned ranges. The aforementionedcontent of Cr forms a strong passivation film. By this means the SSCresistance of the steel material in a sour environment is enhanced. Theaforementioned content of Mo forms Mo sulfides on the passivation film,and thereby inhibits contact between the passivation film and hydrogensulfide ions (HS⁻). As a result, the SSC resistance of the steelmaterial in a sour environment is enhanced. The aforementioned contentof Cu forms Cu sulfides on the passivation film, and thereby inhibitscontact between the passivation film and hydrogen sulfide ions (HS⁻). Asa result, the SSC resistance of the steel material in a sour environmentis enhanced.

However, Cr, Mo, and Cu are elements that easily segregate. In theaforementioned chemical composition, the content of Cr is 10.00 to14.00% which is high, the content of Mo is 1.50 to 3.00% which is alsohigh, and the content of Cu is more than 1.00 to 3.50% which is alsohigh. Therefore, there is a possibility that Cr, Mo, and Cu willsegregate. If Cr, Mo, and Cu segregate, there is a possibility that theSSC resistance in a sour environment will be low.

Thus the present inventors investigated the relation between the degreeof segregation of Cr, Mo, and Cu and the SSC resistance in a sourenvironment with respect to a martensitic stainless steel materialhaving the aforementioned chemical composition and having a yieldstrength of 110 ksi or more.

First, the present inventors conducted studies regarding locations wheresegregation is likely to occur in the steel material. FIG. 1 is across-sectional diagram (transverse cross-sectional diagram) along adirection perpendicular to a longitudinal direction (rolling direction)of a cylindrical billet (round billet) 100 that is the starting materialfor a seamless steel pipe. Referring to FIG. 1 , it has been found thata segregation region SE is likely to be present at the center part inthe transverse cross-section of the billet 100. In the segregationregion SE, Cr, Mo, and Cu easily segregate. Therefore, it was morelikely for Cr segregation, Mo segregation, and Cu segregation to occurin the segregation region SE than in regions other than the segregationregion SE. In addition, when the billet 100 illustrated in FIG. 1 wassubjected to piercing-rolling to be made into a martensitic stainlesssteel material that is a seamless steel pipe, a cross sectionperpendicular to the rolling direction of the seamless steel pipe was asillustrated in FIG. 2 . Specifically, in a transverse cross-section ofthe seamless steel pipe, a segregation region SE was present thatextended in a circumferential direction in a vicinity of an innersurface IS of the seamless steel pipe.

Based on the results of the studies described above, the presentinventors initially considered that, in a martensitic stainless steelmaterial having the aforementioned chemical composition, a yieldstrength of 110 ksi or more and excellent SSC resistance in a sourenvironment can be compatibly obtained if differences between a Crconcentration, a Mo concentration and a Cu concentration in thesegregation region SE that exists in the vicinity of the inner surfaceIS of a seamless steel pipe and a Cr concentration, a Mo concentrationand a Cu concentration in a region other than the segregation region SE,for example, a vicinity of an outer surface OS in FIG. 2 is made small.That is, the present inventors considered that if segregation within amacroscopic region in the steel material can be suppressed, a yieldstrength of 110 ksi or more and excellent SSC resistance in a sourenvironment can be compatibly obtained in a martensitic stainless steelmaterial having the aforementioned chemical composition.

However, in a martensitic stainless steel material having theaforementioned chemical composition, even when differences between theCr concentration, the Mo concentration and the Cu concentration in thesegregation region SE and the Cr concentration, the Mo concentration andthe Cu concentration in regions other than the segregation region SEwere kept small, when the yield strength was made 110 ksi or more, insome cases the SSC resistance was still low.

Therefore, rather than attempting to reduce segregation within amacroscopic region consisting of the segregation region SE and theregions other than the segregation region SE, the present inventorsfocused their attention on microscopic regions within the segregationregion SE, and investigated making the Cr concentration distribution,the Mo concentration distribution, and the Cu concentration distributionwithin the microscopic regions sufficiently uniform.

If the Cr concentration distribution, the Mo concentration distribution,and the Cu concentration distribution within microscopic regions can bemade sufficiently uniform, the Cr concentration distribution, the Moconcentration distribution, and the Cu concentration distribution of thesteel material as a whole will also be sufficiently uniform. As aresult, there is a possibility that a yield strength of 110 ksi or moreand excellent SSC resistance in a sour environment can be compatiblyobtained.

Therefore, instead of focusing their attention on segregation in themacroscopic region, the present inventors focused on microscopic regionswithin the segregation region SE and conducted further studies regardingthe relation between the SSC resistance of the steel material having ayield strength of 110 ksi or more and the Cr concentration distribution,Mo concentration distribution, and Cu concentration distribution.

Specifically, referring to FIG. 3 , in a case where the martensiticstainless steel material was a seamless steel pipe, in a cross sectionincluding a rolling direction L and a wall thickness direction T of theseamless steel pipe, an arbitrary two points at positions at a depth of2 mm from the inner surface IS were defined as two center points P1. Thetwo center points P1 were positions which corresponded to thesegregation region SE illustrated in FIG. 2 .

FIG. 4 is an enlarged view of a vicinity of the two center points P1 inFIG. 3 . Referring to FIG. 4 , two line segments of 1000 μm extending inthe wall thickness direction T that centered on the respective centerpoints P1 were defined as line segments LS. The two line segments LScorresponded to the interior of the segregation region SE, and weremicroscopic regions. On each line segment LS, point analysis usingenergy dispersive X-ray spectroscopy (EDS) was performed at measurementpositions at a pitch of 1 μm, and the Cr concentration (mass %), Moconcentration (mass %), and Cu concentration (mass %) at eachmeasurement position were determined. In the point analysis, theaccelerating voltage was set to 20 kV.

The following items were defined based on the determined Crconcentrations.

-   -   (A) An average value of all of the Cr concentrations determined        at all of the measurement positions on the two line segments LS        was defined as [Cr]_(ave).    -   (B) A sample standard deviation of all of the Cr concentrations        determined at all of the measurement positions on the two line        segments LS was defined as σ_(Cr).    -   (C) Based on the so-called three sigma rule, among all of the Cr        concentrations determined at all of the measurement positions on        the two line segments LS, an average value of the Cr        concentrations included within a range of [Cr]_(ave)±3σ_(Cr) was        defined as [Cr*]_(ave).    -   (D) Among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) was defined as [Cr*]_(max).    -   (E) Among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) was defined as [Cr*]_(min).

Similarly, the following items were defined based on the determined Moconcentrations.

-   -   (F) An average value of all of the Mo concentrations determined        at all of the measurement positions on the two line segments LS        was defined as [Mo]_(ave).    -   (G) A sample standard deviation of all of the Mo concentrations        determined at all of the measurement positions on the two line        segments LS was defined as σ_(Mo).    -   (H) Based on the three sigma rule, among all of the Mo        concentrations determined at all of the measurement positions on        the two line segments LS, an average value of the Mo        concentrations included within a range of [Mo]_(ave)±3σ_(Mo) was        defined as [Mo*]_(ave).    -   (I) Among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) was defined as [Mo*]_(max).    -   (J) Among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) was defined as [Mo*]_(min).

Similarly, the following items were defined based on the determined Cuconcentrations.

-   -   (K) An average value of all of the Cu concentrations determined        at all of the measurement positions on the two line segments LS        was defined as [Cu]_(ave).    -   (L) A sample standard deviation of all of the Cu concentrations        determined at all of the measurement positions on the two line        segments LS was defined as σ_(Cu).    -   (M) Based on the three sigma rule, among all of the Cu        concentrations determined at all of the measurement positions on        the two line segments LS, an average value of the Cu        concentrations included within a range of [Cu]_(ave)±3σ_(Cu) was        defined as [Cu*]_(ave).    -   (N) Among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) was defined as [Cu*]_(max).    -   (O) Among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) was defined as [Cu*]_(min).

Based on the items determined in the above (A) to (O), a degree of Crsegregation ΔCr defined by Formula (1) was determined, a degree of Mosegregation ΔMo defined by Formula (2) was determined, and a degree ofCu segregation ΔCu defined by Formula (3) was determined.

ΔCr=([Cr*]_(max)−[Cr*]_(min))/[Cr*]_(ave)  (1)

ΔMo=([Mo*]_(max)−[Mo*]_(min))/[Mo*]_(ave)  (2)

ΔCu=([Cu*]_(max)−[Cu*]_(min))/[Cu*]_(ave)  (3)

The degree of Cr segregation ΔCr defined by Formula (1) means the degreeof Cr segregation within microscopic regions in the segregation regionSE. The degree of Mo segregation ΔMo defined by Formula (2) means thedegree of Mo segregation within microscopic regions in the segregationregion SE. The degree of Cu segregation ΔCu defined by Formula (3) meansthe degree of Cu segregation within microscopic regions in thesegregation region SE.

The present inventors considered that if the degree of Cr segregationΔCr, the degree of Mo segregation ΔMo, and the degree of Cu segregationΔCu in these microscopic regions can be reduced, the Cr concentrationdistribution, the Mo concentration distribution, and the Cuconcentration distribution in the steel material as a whole will beclose to being sufficiently uniform. Further, the present inventorsconsidered that if the total value of the degree of Cr segregation ΔCr,the degree of Mo segregation ΔMo, and the degree of Cu segregation ΔCuis kept sufficiently low, excellent SSC resistance in a sour environmentwill be obtained even when the steel material has a yield strength of110 ksi or more.

Based on the technical idea described above, on the premise that thesteel material has the aforementioned chemical composition, the presentinventors investigated the relation between the SSC resistance and thetotal value of the degree of Cr segregation ΔCr, the degree of Mosegregation ΔMo, and the degree of Cu segregation ΔCu in microscopicregions within the segregation region SE in the steel material. As aresult, the present inventors discovered that in a martensitic stainlesssteel material having the aforementioned chemical composition, in a casewhere the degree of Cr segregation ΔCr defined by Formula (1), thedegree of Mo segregation ΔMo defined by Formula (2), and the degree ofCu segregation ΔCu defined by Formula (3) satisfy Formula (4), a yieldstrength of 110 ksi grade and excellent SSC resistance in a sourenvironment can be compatibly obtained.

ΔCr+ΔMo+ΔCu≤A  (4)

Here, in a case where the yield strength is 758 to less than 862 MPa, Ain Formula (4) is 0.70, and in a case where the yield strength is 862MPa or more, A in Formula (4) is 0.50.

The martensitic stainless steel material according to the presentdisclosure was completed based on the technical idea described above,and is as follows.

[1]

A martensitic stainless steel material that is a seamless steel pipe ora round steel bar, having a chemical composition consisting of, in mass%:

-   -   C: 0.030% or less,    -   Si: 1.00% or less,    -   Mn: 1.00% or less,    -   P: 0.030% or less,    -   S: 0.0050% or less,    -   Ni: 5.00 to 7.00%,    -   Cr: 10.00 to 14.00%,    -   Mo: 1.50 to 3.00%,    -   Al: 0.005 to 0.050%,    -   V: 0.01 to 0.30%,    -   N: 0.0030 to 0.0500%,    -   Ti: 0.020 to 0.150%,    -   Cu: more than 1.00 to 3.50%,    -   Co: 0.50% or less,    -   B: 0 to 0.0050%,    -   Ca: 0 to 0.0050%,    -   Mg: 0 to 0.0050%,    -   rare earth metal (REM): 0 to 0.0050%,    -   Nb: 0 to 0.15%,    -   W: 0 to 0.20%, and    -   the balance: Fe and impurities,    -   wherein:    -   a yield strength is 758 MPa or more;    -   in a case where the martensitic stainless steel material is the        seamless steel pipe,    -   when, in a cross section including a rolling direction and a        wall thickness direction of the seamless steel pipe, an        arbitrary two points at positions at a depth of 2 mm from an        inner surface are defined as two center points P1, and two line        segments of 1000 μm extending in the wall thickness direction        with each center point P1 as a center are defined as two line        segments LS, energy dispersive X-ray spectroscopy is performed        at measurement positions at a pitch of 1 μm on each line segment        LS, and a Cr concentration, a Mo concentration, and a Cu        concentration at each measurement position are determined;    -   in a case where the martensitic stainless steel material is the        round steel bar,    -   when, in a cross section including a rolling direction and a        radial direction of the round steel bar, an arbitrary two points        on a central axis of the round steel bar are defined as two        center points P1, and two line segments of 1000 μm extending in        the radial direction with each center point P1 as a center are        defined as two line segments LS, energy dispersive X-ray        spectroscopy is performed at measurement positions at a pitch of        1 μm on each line segment LS, and a Cr concentration, a Mo        concentration, and a Cu concentration at each measurement        position are determined; and    -   when:    -   an average value of all of the Cr concentrations determined at        all of the measurement positions on the two line segments LS is        defined as [Cr]_(ave),    -   a sample standard deviation of all of the Cr concentrations        determined at all of the measurement positions on the two line        segments LS is defined as σ_(Cr),    -   among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, an average        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) is defined as [Cr*]_(ave),    -   among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) is defined as [Cr*]_(max),    -   among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) is defined as [Cr*]_(min),    -   an average value of all of the Mo concentrations determined at        all of the measurement positions on the two line segments LS is        defined as [Mo]_(ave),    -   a sample standard deviation of all of the Mo concentrations        determined at all of the measurement positions on the two line        segments LS is defined as σ_(Mo),    -   among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, an average        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) is defined as [Mo*]_(ave),    -   among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) is defined as [Mo*]_(max),    -   among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) is defined as [Mo*]_(min),    -   an average value of all of the Cu concentrations determined at        all of the measurement positions on the two line segments LS is        defined as [Cu]_(ave),    -   a sample standard deviation of all of the Cu concentrations        determined at all of the measurement positions on the two line        segments LS is defined as σ_(Cu),    -   among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, an average        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) is defined as [Cu*]_(ave),    -   among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) is defined as [Cu*]_(max), and    -   among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) is defined as [Cu*]_(min),    -   a degree of Cr segregation ΔCr defined by Formula (1), a degree        of Mo segregation ΔMo defined by Formula (2), and a degree of Cu        segregation ΔCu defined by Formula (3) satisfy Formula (4):

ΔCr=([Cr*]_(max)−[Cr*]_(min))/[Cr*]_(ave)  (1)

ΔMo=([Mo*]_(max)−[Mo*]_(min))/[Mo*]_(ave)  (2)

ΔCu=([Cu*]_(max)−[Cu*]_(min))/[Cu*]_(ave)  (3)

ΔCr+ΔMo+ΔCu≤A  (4)

where, in a case where the yield strength is 758 to less than 862 MPa, Ain Formula (4) is 0.70, and in a case where the yield strength is 862MPa or more, A in Formula (4) is 0.50.

Here, the term “round steel bar” means a steel bar in which a crosssection perpendicular to a longitudinal direction is a circular shape.

[2]

The martensitic stainless steel material according to [1], wherein thechemical composition contains one or more elements selected from thegroup consisting of:

-   -   B: 0.0001 to 0.0050%,    -   Ca: 0.0001 to 0.0050%,    -   Mg: 0.0001 to 0.0050%,    -   rare earth metal (REM): 0.0001 to 0.0050%,    -   Nb: 0.01 to 0.15%, and    -   W: 0.01 to 0.20%.

Hereunder, the martensitic stainless steel material of the presentembodiment is described in detail. The symbol “%” in relation to anelement means mass % unless otherwise stated.

[Chemical Composition]

The chemical composition of the martensitic stainless steel material ofthe present embodiment contains the following elements.

C: 0.030% or Less

Carbon (C) is unavoidably contained. That is, the content of C is morethan 0%. C increases hardenability of the steel material and thusincreases the strength of the steel material. However, if the content ofC is more than 0.030%, C will easily combine with Cr to form Crcarbides. As a result, even if the contents of other elements are withinthe range of the present embodiment, the SSC resistance of the steelmaterial will be likely to decrease.

Accordingly, the content of C is to be 0.030% or less. A preferablelower limit of the content of C is 0.001%, more preferably is 0.003%,and further preferably is 0.005%. A preferable upper limit of thecontent of C is 0.025%, more preferably is 0.020%, and furtherpreferably is 0.015%.

Si: 1.00% or Less

Silicon (Si) is unavoidably contained. That is, the content of Si ismore than 0%. Si deoxidizes steel. However, if the content of Si is morethan 1.00%, the hot workability of the steel material will decrease evenif the contents of other elements are within the range of the presentembodiment.

Accordingly, the content of Si is to be 1.00% or less. A preferablelower limit of the content of Si is 0.05%, more preferably is 0.10%,further preferably is 0.15%, and further preferably is 0.20%. Apreferable upper limit of the content of Si is 0.70%, more preferably is0.50%, further preferably is 0.45%, and further preferably is 0.40%.

Mn: 1.00% or Less

Manganese (Mn) is unavoidably contained. That is, the content of Mn ismore than 0%. Mn increases hardenability of steel material and thusincreases the strength of the steel material. However, if the content ofMn is more than 1.00%, even if the contents of other elements are withinthe range of the present embodiment, Mn will form coarse inclusions andcause toughness of the steel material to decrease.

Accordingly, the content of Mn is to be 1.00% or less. A preferablelower limit of the content of Mn is 0.10%, more preferably is 0.20%, andfurther preferably is 0.25%. A preferable upper limit of the content ofMn is 0.80%, more preferably is 0.60%, and further preferably is 0.50%.

P: 0.030% or Less

Phosphorus (P) is an impurity that is unavoidably contained. That is,the content of P is more than 0%. If the content of P is more than0.030%, even if the contents of other elements are within the range ofthe present embodiment, P will segregate at grain boundaries and causetoughness of the steel material to markedly decrease.

Accordingly, the content of P is to be 0.030% or less. A preferableupper limit of the content of P is 0.025%, and more preferably is0.020%. The content of P is preferably as low as possible. However,excessively reducing the content of P will significantly increase theproduction cost. Therefore, when taking industrial production intoconsideration, a preferable lower limit of the content of P is 0.001%,more preferably is 0.002%, and further preferably is 0.005%.

S: 0.0050% or Less

Sulfur (S) is an impurity that is unavoidably contained. That is, thecontent of S is more than 0%. If the content of S is more than 0.0050%,S will excessively segregate at grain boundaries, and an excessivelylarge amount of MnS that is an inclusion will form. In such a case,toughness and hot workability of the steel material will markedlydecrease even if the contents of other elements are within the range ofthe present embodiment.

Accordingly, the content of S is to be 0.0050% or less. A preferableupper limit of the content of S is 0.0030%, more preferably is 0.0020%,and further preferably is 0.0015%. The content of S is preferably as lowas possible. However, excessively reducing the content of S willsignificantly increase the production cost. Therefore, when takingindustrial production into consideration, a preferable lower limit ofthe content of S is 0.0001%, more preferably is 0.0002%, and furtherpreferably is 0.0004%.

Ni: 5.00 to 7.00%

Nickel (Ni) forms sulfides on a passivation film in a sour environment.The Ni sulfides inhibit chloride ions (Cl⁻) and hydrogen sulfide ions(HS⁻) from coming into contact with the passivation film. Consequently,it is difficult for the passivation film to be destroyed by chlorideions and hydrogen sulfide ions. As a result, Ni increases the SSCresistance of the steel material in a sour environment. Ni is also anaustenite-forming element. Therefore, Ni causes the microstructure ofthe steel material after quenching to become martensitic. If the contentof Ni is less than 5.00%, even if the contents of other elements arewithin the range of the present embodiment, the aforementioned effectswill not be sufficiently obtained. On the other hand, if the content ofNi is more than 7.00%, the aforementioned effects will be saturated andthe production cost will increase.

Accordingly, the content of Ni is to be 5.00 to 7.00%. A preferablelower limit of the content of Ni is 5.10%, more preferably is 5.15%, andfurther preferably is 5.20%. A preferable upper limit of the content ofNi is 6.50%, more preferably is 6.40%, further preferably is 6.30%, andfurther preferably is 6.20%.

Cr: 10.00 to 14.00%

Chromium (Cr) forms a passivation film on the surface of the steelmaterial in a sour environment, and thereby improves the SSC resistanceof the steel material. If the content of Cr is less than 10.00%, theaforementioned effect will not be sufficiently obtained even if thecontents of other elements are within the range of the presentembodiment. On the other hand, if the content of Cr is more than 14.00%,Cr carbides, intermetallic compounds containing Cr, and Cr oxides willexcessively form. In such a case the SSC resistance of the steelmaterial will decrease even if the contents of other elements are withinthe range of the present embodiment.

Accordingly, the content of Cr is to be 10.00 to 14.00%. A preferablelower limit of the content of Cr is 10.05%, more preferably is 10.10%,further preferably is 10.50%, and further preferably is 11.00%. Apreferable upper limit of the content of Cr is 13.70%, more preferablyis 13.50%, further preferably is 13.40%, and further preferably is13.30%.

Mo: 1.50 to 3.00%

Molybdenum (Mo) forms sulfides on a passivation film in a sourenvironment. The Mo sulfides inhibit chloride ions (Cl⁻) and hydrogensulfide ions (HS⁻) from coming into contact with the passivation film.Consequently, it is difficult for the passivation film to be destroyedby chloride ions and hydrogen sulfide ions. As a result, Mo increasesthe SSC resistance of the steel material in a sour environment. If thecontent of Mo is less than 1.50%, this effect will not be sufficientlyobtained even if the contents of other elements are within the range ofthe present embodiment. On the other hand, if the content of Mo is morethan 3.00%, the aforementioned effect will be saturated and theproduction cost will increase.

Accordingly, the content of Mo is to be 1.50 to 3.00%. A preferablelower limit of the content of Mo is 1.70%, more preferably is 1.80%,further preferably is 1.90%, and further preferably is 2.00%. Apreferable upper limit of the content of Mo is 2.95%, more preferably is2.90%, further preferably is 2.85%, and further preferably is 2.80%.

Al: 0.005 to 0.050%

Aluminum (Al) deoxidizes steel. If the content of Al is less than0.005%, the aforementioned effect will not be sufficiently obtained evenif the contents of other elements are within the range of the presentembodiment. On the other hand, if the content of Al is more than 0.050%,even if the contents of other elements are within the range of thepresent embodiment, coarse Al oxides will form and the toughness of thesteel material will decrease.

Accordingly, the content of Al is to be 0.005 to 0.050%. A preferablelower limit of the content of Al is 0.007%, more preferably is 0.010%,and further preferably is 0.015%. A preferable upper limit of thecontent of Al is 0.047%, more preferably is 0.043%, and furtherpreferably is 0.040%. In the present description, the term “content ofAl” means the content of sol. Al (acid-soluble Al).

V: 0.01 to 0.30%

Vanadium (V) forms V precipitates such as carbides, nitrides, andcarbo-nitrides in the steel material. The V precipitates increase thestrength of the steel material. If the content of V is less than 0.01%,the aforementioned effect will not be sufficiently obtained even if thecontents of other elements are within the range of the presentembodiment. On the other hand, if the content of V is more than 0.30%, Vprecipitates will excessively form and the strength of the steelmaterial will become excessively high. In such a case, the SSCresistance of the steel material will decrease even if the contents ofother elements are within the range of the present embodiment.

Accordingly, the content of V is to be 0.01 to 0.30%. A preferable lowerlimit of the content of V is 0.02%, and more preferably is 0.03%. Apreferable upper limit of the content of V is 0.25%, more preferably is0.20%, further preferably is 0.15%, further preferably is 0.10%, andfurther preferably is 0.08%.

N: 0.0030 to 0.0500%

Nitrogen (N) improves pitting resistance of the steel material andincreases the SSC resistance of the steel material. If the content of Nis less than 0.0030%, the aforementioned effect will not be sufficientlyobtained even if the contents of other elements are within the range ofthe present embodiment. On the other hand, if the content of N is morethan 0.0500%, coarse TiN will form. In such a case, the SSC resistanceof the steel material will decrease even if the contents of otherelements are within the range of the present embodiment.

Accordingly, the content of N is to be 0.0030 to 0.0500%. A preferablelower limit of the content of N is 0.0033%, more preferably is 0.0035%,and further preferably is 0.0038%. A preferable upper limit of thecontent of N is 0.0400%, more preferably is 0.0300%, further preferablyis 0.0200%, further preferably is 0.0100%, further preferably is0.0080%, and further preferably is 0.0070%.

Ti: 0.020 to 0.150%

Titanium (Ti) combines with C or N to form Ti precipitates that arecarbides or nitrides. The Ti precipitates suppress coarsening of grainsby the pinning effect. As a result, the strength of the steel materialincreases. In addition, an excessive increase in strength due toexcessive formation of V precipitates is suppressed by formation of theTi precipitates. As a result, the SSC resistance of the steel materialincreases. Here, the term “V precipitates” refers to carbides, nitrides,carbo-nitrides and the like. If the content of Ti is less than 0.020%,the aforementioned effects will not be sufficiently obtained even if thecontents of other elements are within the range of the presentembodiment. On the other hand, if the content of Ti is more than 0.150%,the aforementioned effects will be saturated. Furthermore, if thecontent of Ti is more than 0.150%, Ti carbides or Ti nitrides willexcessively form, and toughness of the steel material will decrease.

Accordingly, the content of Ti is to be 0.020 to 0.150%. A preferablelower limit of the content of Ti is 0.030%, more preferably is 0.040%,and further preferably is 0.050%. A preferable upper limit of thecontent of Ti is 0.140%, and more preferably is 0.130%.

Cu: More than 1.00 to 3.50%

Copper (Cu) forms sulfides on a passivation film in a sour environment.The Cu sulfides inhibit chloride ions (Cl⁻) and hydrogen sulfide ions(HS⁻) from coming into contact with the passivation film. Consequently,it is difficult for the passivation film to be destroyed by chlorideions and hydrogen sulfide ions. As a result, Cu increases the SSCresistance of the steel material in a sour environment. If the contentof Cu is less than 1.00%, this effect will not be sufficiently obtainedeven if the contents of other elements are within the range of thepresent embodiment. On the other hand, if the content of Cu is more than3.50%, hot workability of the steel material will decrease even if thecontents of other elements are within the range of the presentembodiment.

Accordingly, the content of Cu is to be more than 1.00 to 3.50%. Apreferable lower limit of the content of Cu is 1.40%, more preferably is1.50%, further preferably is 1.60%, further preferably is 1.70%, andfurther preferably is 1.80%. A preferable upper limit of the content ofCu is 3.30%, more preferably is 3.10%, and further preferably is 3.00%.

Co: 0.50% or Less

Cobalt (Co) is unavoidably contained. That is, the content of Co is morethan 0%. In a sour environment, Co forms sulfides on a passivation film.The Co sulfides inhibit chloride ions (Cl⁻) and hydrogen sulfide ions(HS⁻) from coming into contact with the passivation film. Consequently,it is difficult for the passivation film to be destroyed by chlorideions and hydrogen sulfide ions. As a result, Co increases the SSCresistance of the steel material. Co also suppresses the formation ofretained austenite, and suppresses the occurrence of variations in thestrength of the steel material. However, if the content of Co is morethan 0.50%, toughness of the steel material will decrease even if thecontents of other elements are within the range of the presentembodiment.

Accordingly, the content of Co is to be 0.50% or less. A preferablelower limit of the content of Co is 0.01%, more preferably is 0.05%,further preferably is 0.10%, and further preferably is 0.15%. Apreferable upper limit of the content of Co is 0.45%, more preferably is0.40%, further preferably is 0.35%, and further preferably is 0.30%.

The balance of the chemical composition of the martensitic stainlesssteel material according to the present embodiment is Fe and impurities.Here, the term “impurities” refers to elements which, during industrialproduction of the martensitic stainless steel material, are mixed infrom ore or scrap that is used as the raw material, or from theproduction environment or the like, and which are not intentionallycontained but are allowed within a range that does not adverselyinfluence the advantageous effects of the martensitic stainless steelmaterial of the present embodiment.

[Regarding Optional Elements]

The chemical composition of the martensitic stainless steel materialaccording to the present embodiment may further contain, in lieu of apart of Fe, one or more optional elements selected from the followinggroup.

-   -   B: 0 to 0.0050%    -   Ca: 0 to 0.0050%    -   Mg: 0 to 0.0050%    -   Rare earth metal (REM): 0 to 0.0050%    -   Nb: 0 to 0.15%    -   W: 0 to 0.20% Hereunder, these optional elements are described.

[First Group: B, Ca, Mg, and Rare Earth Metal (REM)]

The chemical composition of the martensitic stainless steel materialaccording to the present embodiment may further contain one or moreelements selected from the group consisting of B, Ca, Mg, and rare earthmetal (REM) in lieu of a part of Fe. These elements are optionalelements, and each of these elements increases the hot workability ofthe steel material.

B: 0 to 0.0050%

Boron (B) is an optional element, and need not be contained. That is,the content of B may be 0%. When contained, B segregates at austenitegrain boundaries and strengthens the grain boundaries. As a result, hotworkability of the steel material is increased. If even a small amountof B is contained, the aforementioned effect will be obtained to acertain extent. However, if the content of B is more than 0.0050%, Crcarbo-borides will form even if the contents of other elements arewithin the range of the present embodiment. In such a case, toughness ofthe steel material will decrease.

Accordingly, the content of B is to be 0 to 0.0050%. A preferable lowerlimit of the content of B is 0.0001%, and more preferably is 0.0002%. Apreferable upper limit of the content of B is 0.0040%, more preferablyis 0.0030%, further preferably is 0.0020%, further preferably is0.0010%, further preferably is 0.0008%, and further preferably is0.0007%.

Ca: 0 to 0.0050%

Calcium (Ca) is an optional element, and need not be contained. That is,the content of Ca may be 0%. When contained, Ca spheroidizes and/orrefines inclusions, and thereby increases hot workability of the steelmaterial. If even a small amount of Ca is contained, this effect will beobtained to a certain extent. However, if the content of Ca is more than0.0050%, coarse oxides will form. In such a case, toughness of the steelmaterial will decrease even if the contents of other elements are withinthe range of the present embodiment.

Accordingly, the content of Ca is to be 0 to 0.0050%. A preferable lowerlimit of the content of Ca is 0.0001%, more preferably is 0.0005%,further preferably is 0.0010%, and further preferably is 0.0015%. Apreferable upper limit of the content of Ca is 0.0045%, more preferablyis 0.0040%, and further preferably is 0.0035%.

Mg: 0 to 0.0050%

Magnesium (Mg) is an optional element, and need not be contained. Thatis, the content of Mg may be 0%. When contained, similarly to Ca, Mgspheroidizes and/or refines inclusions, and thereby increases hotworkability of the steel material. If even a small amount of Mg iscontained, the aforementioned effect will be obtained to a certainextent. However, if the content of Mg is more than 0.0050%, coarseoxides will form. In such a case, toughness of the steel material willdecrease even if the contents of other elements are within the range ofthe present embodiment.

Accordingly, the content of Mg is to be 0 to 0.0050%. A preferable lowerlimit of the content of Mg is 0.0001%, more preferably is 0.0005%, andfurther preferably is 0.0010%. A preferable upper limit of the contentof Mg is 0.0045%, more preferably is 0.0035%, and further preferably is0.0025%.

Rare Earth Metal (REM): 0 to 0.0050%

Rare earth metal (REM) is an optional element, and need not becontained. That is, the content of REM may be 0%. When contained,similarly to Ca, REM spheroidizes and/or refines inclusions, and therebyincreases hot workability of the steel material. If even a small amountof REM is contained, the aforementioned effect will be obtained to acertain extent. However, if the content of REM is more than 0.0050%,coarse oxides will form. In such a case, toughness of the steel materialwill decrease even if the contents of other elements are within therange of the present embodiment.

Accordingly, the content of REM is to be 0 to 0.0050%. A preferablelower limit of the content of REM is 0.0001%, more preferably is0.0005%, and further preferably is 0.0010%. A preferable upper limit ofthe content of REM is 0.0045%, more preferably is 0.0035%, and furtherpreferably is 0.0025%.

Note that, in the present description the term “REM” means one or moreelements selected from the group consisting of scandium (Sc) which isthe element with atomic number 21, yttrium (Y) which is the element withatomic number 39, and the elements from lanthanum (La) with atomicnumber 57 to lutetium (Lu) with atomic number 71 that are lanthanoids.Further, in the present description the term “content of REM” refers tothe total content of these elements.

[Second Group: Nb and W]

The chemical composition of the martensitic stainless steel materialaccording to the present embodiment may further contain one or moreelements selected from the group consisting of Nb and W in lieu of apart of Fe. These elements are optional elements, and each of theseelements increases the SSC resistance of the steel material.

Nb: 0 to 0.15%

Niobium (Nb) is an optional element, and need not be contained. That is,the content of Nb may be 0%. When contained, Nb forms Nb precipitatesthat are fine carbides, nitrides, or carbo-nitrides. The Nb precipitatesrefine the substructure of the steel material by the pinning effect. Asa result, the SSC resistance of the steel material increases. If even asmall amount of Nb is contained, the aforementioned effect will beobtained to a certain extent. However, if the content of Nb is more than0.15%, Nb precipitates will excessively form. In such a case, the SSCresistance of the steel material will decrease even if the contents ofother elements are within the range of the present embodiment.

Accordingly, the content of Nb is to be 0 to 0.15%. A preferable lowerlimit of the content of Nb is 0.01%, more preferably is 0.02%, andfurther preferably is 0.03%. A preferable upper limit of the content ofNb is 0.14%, more preferably is 0.13%, and further preferably is 0.10%.

W: 0 to 0.20%

Tungsten (W) is an optional element, and need not be contained. That is,the content of W may be 0%. When contained, W stabilizes the passivationfilm in a sour environment. Consequently, it is difficult for thepassivation film to be destroyed by chloride ions and hydrogen sulfideions. As a result, the SSC resistance of the steel material increases.If even a small amount of W is contained, the aforementioned effect willbe obtained to a certain extent. However, if the content of W is morethan 0.20%, W will combine with C, and coarse W carbides will be formed.In such a case, toughness of the steel material will decrease even ifthe contents of other elements are within the range of the presentembodiment.

Accordingly, the content of W is to be 0 to 0.20%. A preferable lowerlimit of the content of W is 0.01%, more preferably is 0.03%, andfurther preferably is 0.05%. A preferable upper limit of the content ofW is 0.18%, and more preferably is 0.16%.

[Regarding Cr Concentration Distribution, Mo Concentration Distribution,and Cu Concentration Distribution in Steel Material]

In the martensitic stainless steel material of the present embodiment,in addition, a degree of Cr segregation ΔCr defined by Formula (1), adegree of Mo segregation ΔMo defined by Formula (2), and a degree of Cusegregation ΔCu defined by Formula (3) satisfy Formula (4):

ΔCr=([Cr*]_(max)−[Cr*]_(min))/[Cr*]_(ave)  (1)

ΔMo=([Mo*]_(max)−[Mo*]_(min))/[Mo*]_(ave)  (2)

ΔCu=([Cu*]_(max)−[Cu*]_(min))/[Cu*]_(ave)  (3)

ΔCr+ΔMo+ΔCu≤A  (4)

where, in a case where the yield strength is 758 to less than 862 MPa, Ain Formula (4) is 0.70, and in a case where the yield strength is 862MPa or more, A in Formula (4) is 0.50.

The degree of Cr segregation ΔCr defined by Formula (1), the degree ofMo segregation ΔMo defined by Formula (2), and the degree of Cusegregation ΔCu defined by Formula (3) are determined by the followingmethod.

[Method for Measuring Degree of Cr Segregation ΔCr, Degree of MoSegregation ΔMo, and Degree of Cu Segregation ΔCu]

Referring to FIG. 3 , in a case where the martensitic stainless steelmaterial is a seamless steel pipe, in a cross section including arolling direction L and a wall thickness direction T of the seamlesssteel pipe, an arbitrary two points at positions at a depth of 2 mm froman inner surface IS are defined as two center points P1. Referring toFIG. 4 , two line segments of 1000 μm extending in the wall thicknessdirection T with each center point P1 as a center are defined as twoline segments LS. On each line segment LS, point analysis using energydispersive X-ray spectroscopy (EDS) is performed at measurementpositions at a pitch of 1 μm, and the Cr concentration (mass %), the Moconcentration (mass %), and the Cu concentration (mass %) at eachmeasurement position are determined. In the point analysis, theaccelerating voltage is set to 20 kV.

Similarly, in a case where the martensitic stainless steel material is around steel bar, referring to FIG. 5 , in a cross section including arolling direction L and a radial direction D of the round steel bar, anarbitrary two points on a central axis CI of the round steel bar aredefined as two center points P1. Two line segments of 1000 μm extendingin the radial direction D with each center point P1 as a center aredefined as two line segments LS. On each line segment LS, point analysisusing EDS is performed at measurement positions at a pitch of 1 μm, andthe Cr concentration (mass %), the Mo concentration (mass %), and the Cuconcentration (mass %) at each measurement position are determined. Inthe point analysis, the accelerating voltage is set to 20 kV.

The following items are defined based on the determined Crconcentrations.

-   -   (A) An average value of all of the Cr concentrations determined        at all of the measurement positions on the two line segments LS        is defined as [Cr]_(ave).    -   (B) A sample standard deviation of all of the Cr concentrations        determined at all of the measurement positions on the two line        segments LS is defined as σ_(Cr).    -   (C) Based on the so-called three sigma rule, among all of the Cr        concentrations determined at all of the measurement positions on        the two line segments LS, an average value of the Cr        concentrations included within a range of [Cr]_(ave)±3σ_(Cr) is        defined as [Cr*]_(ave).    -   (D) Among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) is defined as [Cr*]_(max).    -   (E) Among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) is defined as [Cr*]_(min).

Similarly, the following items are defined based on the determined Moconcentrations.

-   -   (F) An average value of all of the Mo concentrations determined        at all of the measurement positions on the two line segments LS        is defined as [Mo]_(ave).    -   (G) A sample standard deviation of all of the Mo concentrations        determined at all of the measurement positions on the two line        segments LS is defined as σ_(Mo),    -   (H) Based on the three sigma rule, among all of the Mo        concentrations determined at all of the measurement positions on        the two line segments LS, an average value of the Mo        concentrations included within a range of [Mo]_(ave)±3σ_(Mo) is        defined as [Mo*]_(ave),    -   (I) Among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) is defined as [Mo*]_(max).    -   (J) Among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) is defined as [Mo*]_(min).

Similarly, the following items are defined based on the determined Cuconcentrations.

-   -   (K) An average value of all of the Cu concentrations determined        at all of the measurement positions on the two line segments LS        is defined as [Cu]_(ave).    -   (L) A sample standard deviation of all of the Cu concentrations        determined at all of the measurement positions on the two line        segments LS is defined as σ_(Cu).    -   (M) Based on the three sigma rule, among all of the Cu        concentrations determined at all of the measurement positions on        the two line segments LS, an average value of the Cu        concentrations included within a range of [Cu]_(ave)±3σ_(Cu) is        defined as [Cu*]_(ave).    -   (N) Among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) is defined as [Cu*]_(max),    -   (O) Among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) is defined as [Cu*]_(min).

Based on the items determined in the above (A) to (O), the degree of Crsegregation ΔCr defined by Formula (1) is determined, the degree of Mosegregation ΔMo defined by Formula (2) is determined, and the degree ofCu segregation ΔCu defined by Formula (3) is determined.

ΔCr=([Cr*]_(max)−[Cr*]_(min))/[Cr*]_(ave)  (1)

ΔMo=([Mo*]_(max)−[Mo*]_(min))/[Mo*]_(ave)  (2)

ΔCu=([Cu*]_(max)−[Cu*]_(min))/[Cu*]_(ave)  (3)

In the martensitic stainless steel material of the present embodiment,the degree of Cr segregation ΔCr defined by Formula (1), the degree ofMo segregation ΔMo defined by Formula (2), and the degree of Cusegregation ΔCu defined by Formula (3) satisfy Formula (4):

ΔCr+ΔMo+ΔCu≤A  (4)

where, in a case where the yield strength is 758 to less than 862 MPa, Ain Formula (4) is 0.70, and in a case where the yield strength is 862MPa or more, A in Formula (4) is 0.50.

Let a total degree of segregation ΔF be defined as ΔF=ΔCr+ΔMo+ΔCu. Eachline segment LS that is a measurement region for measuring the Crconcentration, the Mo concentration, and the Cu concentration, in otherwords, each line segment LS which extends in the wall thicknessdirection T or the radial direction D and has the center point P1 as itscenter is a region where Cr, Mo, and Cu segregate the most in the steelmaterial. The line segments LS are microscopic regions in the steelmaterial.

Here, a case in which the yield strength of the steel material of thepresent embodiment is 110 ksi grade (758 to less than 862 MPa) will beassumed. In this case, if the total degree of segregation ΔF that is thetotal sum of the degree of Cr segregation ΔCr, the degree of Mosegregation ΔMo, and the degree of Cu segregation ΔCu on the linesegments LS is 0.70 or less, segregation of the Cr concentration, the Moconcentration, and the Cu concentration is sufficiently suppressed evenin the microscopic regions in which the Cr concentration, the Moconcentration, and the Cu concentration are segregated the most. Thismeans that in the entire steel material also, in other words, themacroscopic region of the steel material, the Cr concentration, the Moconcentration, and the Cu concentration are each distributed in asufficiently uniform manner

Similarly, a case in which the yield strength of the steel material ofthe present embodiment is 125 ksi or more (862 MPa or more) will beassumed. In this case, if the total degree of segregation ΔF that is thetotal sum of the degree of Cr segregation ΔCr, the degree of Mosegregation ΔMo, and the degree of Cu segregation ΔCu on the linesegments LS is 0.50 or less, segregation of the Cr concentration, the Moconcentration, and the Cu concentration is sufficiently suppressed evenin the microscopic regions in which the Cr concentration, the Moconcentration, and the Cu concentration are segregated the most. Thismeans that in the entire steel material also, in other words, themacroscopic region of the steel material, the Cr concentration, the Moconcentration, and the Cu concentration are each distributed in asufficiently uniform manner

Accordingly, the total degree of segregation ΔF is to be 0.70 or less ina case where the yield strength of the steel material is 110 ksi grade,and is to be 0.50 or less in a case where the yield strength of thesteel material is 125 ksi or more.

By being composed as described above, the martensitic stainless steelmaterial of the present embodiment can obtain excellent SSC resistancein a sour environment while also having a yield strength of 110 ksi ormore.

When the yield strength of the steel material is 110 ksi grade (758 toless than 862 MPa), a preferable upper limit of ΔF is 0.65, morepreferably is 0.63, further preferably is 0.61, further preferably is0.59, further preferably is 0.57, and further preferably is 0.55.

When the yield strength of the steel material is 125 ksi or more (862MPa or more), a preferable upper limit of ΔF is 0.49, more preferably is0.48, and further preferably is 0.47.

[Microstructure]

The microstructure of the martensitic stainless steel material accordingto the present embodiment is mainly composed of martensite. In thepresent description, the term “martensite” includes not only freshmartensite but also tempered martensite. Moreover, in the presentdescription, the phrase “mainly composed of martensite” means that thevolume ratio of martensite is 80.0% or more in the microstructure.

In the microstructure of the martensitic stainless steel materialaccording to the present embodiment, a preferable lower limit of thevolume ratio of martensite is 85.0%, and more preferably is 90.0%.Further preferably, the microstructure of the steel material is composedof single-phase martensite.

The balance of the microstructure is retained austenite. That is, thevolume ratio of retained austenite is 0 to 20.0% in the martensiticstainless steel material of the present embodiment. The volume ratio ofretained austenite is preferably as low as possible.

On the other hand, in the microstructure, a small amount of retainedaustenite significantly increases the toughness of steel material whilesuppressing the occurrence of a significant decrease in strength.Accordingly, when it is desired to increase toughness, a microstructurethat includes retained austenite may be adopted. However, if the volumeratio of retained austenite is too high, the strength of the steelmaterial will markedly decrease. Accordingly, in a case where themicrostructure of the steel material includes retained austenite, apreferable upper limit of the volume ratio of retained austenite is15.0%, and further preferably is 10.0%.

[Method for Measuring Volume Ratio of Martensite]

The volume ratio (%) of martensite in the microstructure of themartensitic stainless steel material of the present embodiment can beobtained by subtracting the volume ratio (%) of retained austenite,which is obtained by the following method, from 100.0%.

The volume ratio of retained austenite can be obtained by an X-raydiffraction method. Specifically, a test specimen is taken from themartensitic stainless steel material. In a case where the martensiticstainless steel material is a seamless steel pipe, the test specimen istaken from a center portion of the wall thickness of the steel pipe. Ina case where the martensitic stainless steel material is a round steelbar, the test specimen is taken from an R/2 portion, that is, a centerportion of a radius R in a cross section perpendicular to thelongitudinal direction of the round steel bar. Although not particularlylimited, the size of the test specimen is, for example, 15 mm×15 mm×athickness of 2 mm. In this case, the thickness direction of the testspecimen is the wall thickness direction in a case where the martensiticstainless steel material is a seamless steel pipe, and is the radialdirection in a case where the martensitic stainless steel material is around steel bar.

Using the obtained test specimen, the X-ray diffraction intensity ofeach of the (200) plane of α phase, the (211) plane of α phase, the(200) plane of γ phase, the (220) plane of γ phase, and the (311) planeof γ phase is measured to calculate an integrated intensity of eachplane. In the measurement of the X-ray diffraction intensity, the targetof the X-ray diffraction apparatus is Mo (MoKα ray), and the output is50 kV and 40 mA.

After calculation, the volume ratio Vγ (%) of retained austenite iscalculated using Formula (I) for combinations (2×3=6 pairs) of eachplane of the α phase and each plane of the γ phase. Then, an averagevalue of the volume ratios Vγ of retained austenite of the six pairs isdefined as the volume ratio (%) of retained austenite.

Vγ=100/{1+(Iα×Rγ)/(Iγ×Rα)}  (I)

Where, Iα is an integrated intensity of α phase. Rα is acrystallographic theoretical calculation value of α phase. Iγ is anintegrated intensity of γ phase. Rγ is a crystallographic theoreticalcalculation value of γ phase. Note that, in the present description, Rαin the (200) plane of α phase is 15.9, Rα in the (211) plane of α phaseis 29.2, Rγ in the (200) plane of γ phase is 35.5, Rγ in the (220) planeof γ phase is 20.8, and Rγ in the (311) plane of γ phase is 21.8. Notethat the volume ratio of retained austenite is obtained by rounding offthe second decimal place of an obtained numerical value.

Using the volume ratio (%) of retained austenite obtained by the abovedescribed X-ray diffraction method, the volume ratio (vol. %) ofmartensite of the microstructure of the martensitic stainless steelmaterial is obtained by the following Formula.

Volume ratio of martensite=100.0−volume ratio of retained austenite (%)

[Yield Strength]

The yield strength of the martensitic stainless steel material of thepresent embodiment is 110 ksi or more, that is, 758 MPa or more.

In the present description, the yield strength means 0.2% offset proofstress (MPa) which is obtained by a tensile test at normal temperature(24±3° C.) in conformity with ASTM E8/E8M (2013). Specifically, theyield strength is obtained by the following method.

In a case where the martensitic stainless steel material is a seamlesssteel pipe, a tensile test specimen is taken from the center portion ofthe wall thickness of the steel pipe. In a case where the martensiticstainless steel material is a round steel bar, a tensile test specimenis taken from the R/2 portion. The tensile test specimen is, forexample, a round bar tensile test specimen having a parallel portiondiameter of 6.0 mm and a parallel portion length of 40.0 mm. Thelongitudinal direction of the parallel portion of the round bar tensiletest specimen is made parallel with the rolling direction (longitudinaldirection) of the martensitic stainless steel material.

A tensile test is conducted at normal temperature (24±3° C.) inconformity with ASTM E8/E8M (2013) using the round bar tensile testspecimen to obtain 0.2% offset proof stress (MPa). The obtained 0.2%offset proof stress is defined as the yield strength (MPa).

Although an upper limit of the yield strength of the martensiticstainless steel material of the present embodiment is not particularlylimited, when the contents of the elements are within the ranges of thechemical composition described above, the upper limit of the yieldstrength is, for example, 1000 MPa (145 ksi), and preferably is 965 MPa(140 ksi).

The yield strength of the martensitic stainless steel material of thepresent embodiment may be 110 ksi grade (758 to less than 862 MPa), ormay be 125 ksi or more (862 MPa or more).

In a case where the yield strength of the martensitic stainless steelmaterial of the present embodiment is made 110 ksi grade, a preferablelower limit of the yield strength is 765 MPa, more preferably is 770MPa, further preferably is 775 MPa, and further preferably is 780 MPa. Apreferable upper limit of the yield strength of the martensiticstainless steel material of the present embodiment is 860 MPa, and morepreferably is 855 MPa.

In a case where the yield strength of the martensitic stainless steelmaterial of the present embodiment is made 125 ksi or more, a preferablelower limit of the yield strength is 870 MPa, more preferably is 880MPa, further preferably is 890 MPa, and further preferably is 900 MPa.

[SSC Resistance of Steel Material]

The SSC resistance of the steel material according to the presentembodiment can be evaluated by a SSC resistance evaluation testconducted in accordance with NACE TM0177-2005 Method A.

An SSC resistance evaluation test method that is in accordance with NACETM0177-2005 Method A is as follows. A round bar specimen is taken fromthe martensitic stainless steel material according to the presentembodiment. If the martensitic stainless steel material is a steel pipe,the round bar specimen is taken from the center portion of the wallthickness. If the martensitic stainless steel material is a round steelbar, the round bar specimen is taken from the R/2 portion. The size ofthe round bar specimen is not particularly limited. The round barspecimen, for example, has a size in which the diameter of the parallelportion is 6.35 mm, and the length of the parallel portion is 25.4 mmNote that, the longitudinal direction of the round bar specimen is madeparallel with the rolling direction (longitudinal direction) of themartensitic stainless steel material.

An aqueous solution containing 20 mass % of sodium chloride in which thepH is 4.0 is adopted as the test solution. A stress equivalent to 90% ofthe actual yield stress is applied to the round bar specimen. The testsolution at 24° C. is poured into a test vessel so that the round barspecimen to which the stress has been applied is immersed therein, andthis is adopted as a test bath. After degassing the test bath, a gaseousmixture consisting of H₂S at 0.10 bar and CO₂ at 0.90 bar is blown intothe test bath so that the test bath is saturated with H₂S gas. The testbath in which the H₂S gas is saturated is held at 24° C. for 720 hours.After the test specimen has been held for 720 hours, the surface of thetest specimen is observed with a magnifying glass with a magnificationof ×10 to check for the presence of cracking. If a place is found wherecracking is suspected in the observation with a magnifying glass, across section at the place where cracking is suspected is observed withan optical microscope with a magnification of ×100 to confirm whether ornot there is cracking.

The martensitic stainless steel material of the present embodiment hasexcellent SSC resistance. Specifically, in the martensitic stainlesssteel material of the present embodiment, in the aforementioned SSCresistance evaluation test conducted in accordance with NACE TM0177-2005Method A, cracking is not confirmed after 720 hours elapses. In thepresent description, the phrase “cracking is not confirmed” means thatcracking is not confirmed as a result of observing the test specimenafter the test with a magnifying glass with a magnification of ×10 andan optical microscope with a magnification of ×100.

[Shape and Uses of Martensitic Stainless Steel Material]

The martensitic stainless steel material according to the presentembodiment is a seamless steel pipe or a round steel bar (solidmaterial). In a case where the martensitic stainless steel material is aseamless steel pipe, the martensitic stainless steel material is a steelpipe for oil country tubular goods. The term “steel pipe for oil countrytubular goods” means a steel pipe that is to be used in oil countrytubular goods. Oil country tubular goods are, for example, a casingpipe, a tubing pipe, and a drilling pipe which are used for drilling ofan oil well or a gas well, collection of crude oil or natural gas, andthe like.

In a case where the martensitic stainless steel material is a roundsteel bar, for example, the martensitic stainless steel material is tobe used for a downhole member.

As described above, in the martensitic stainless steel material of thepresent embodiment, the content of each element in the chemicalcomposition is within the range of the present embodiment, and in amicroscopic segregation region (line segment LS), a degree of Crsegregation ΔCr defined by Formula (1), a degree of Mo segregation ΔModefined by Formula (2), and a degree of Cu segregation ΔCu defined byFormula (3) satisfy Formula (4). That is, in a microscopic segregationregion (line segment LS) in the steel material also, the Crconcentration distribution, the Mo concentration distribution, and theCu concentration distribution are sufficiently uniform. Therefore, themartensitic stainless steel material of the present embodiment canobtain excellent SSC resistance in a sour environment while also havinga yield strength of 110 ksi grade.

[Production Method]

An example of a method for producing the martensitic stainless steelmaterial of the present embodiment will now be described. Note that, theproduction method described hereunder is an example, and a method forproducing the martensitic stainless steel material of the presentembodiment is not limited to this production method. That is, as long asthe martensitic stainless steel material of the present embodiment thatis composed as described above can be produced, a method for producingthe martensitic stainless steel material is not limited to theproduction method described hereunder. However, the production methoddescribed hereunder is a favorable method for producing the martensiticstainless steel material of the present embodiment.

One example of a method for producing the martensitic stainless steelmaterial of the present embodiment includes the following processes.

-   -   (1) Starting material preparation process    -   (2) Blooming process    -   (3) Steel material production process    -   (4) Heat treatment process

Hereunder, each process is described in detail.

[(1) Starting Material Preparation Process]

In the starting material preparation process, molten steel in which thecontent of each element in the chemical composition is within the rangeof the present embodiment is produced by a well-known steel-makingmethod. A cast piece is produced by a continuous casting process usingthe produced molten steel. Here, the cast piece is a bloom or a billet.Instead of the cast piece, an ingot may be produced by an ingot-makingprocess using the aforementioned molten steel. The starting material(bloom or ingot) is produced by the above described production process.

[(2) Blooming Process]

In the blooming process, the starting material (bloom or ingot) issubjected to hot rolling using a blooming mill to thereby produce abillet. The blooming process includes the following processes.

-   -   (21) Starting material heating process    -   (22) Hot working process

Hereunder, each process is described in detail.

[(21) Starting Material Heating Process]

In the starting material heating process, the starting material isheated in a bloom reheating furnace. The in-furnace temperature of thebloom reheating furnace and the residence time of the starting materialin the bloom reheating furnace are as follows.

In-furnace temperature of bloom reheating furnace: 1200 to 1350° C.

Holding time in bloom reheating furnace: 200 to 400 minutes

Here, the term “holding time” refers to the in-furnace residence timefrom a time point at which the in-furnace temperature of the heatingfurnace reaches a predetermined temperature.

The aforementioned range of the in-furnace temperature (° C.) of thebloom reheating furnace is a well-known range. The aforementioned rangeof the holding time (minutes) at the bloom reheating furnace is also awell-known range. If the in-furnace temperature of the bloom reheatingfurnace is 1200 to 1350° C., and the holding time in the bloom reheatingfurnace is 200 to 400 minutes, the hot workability of the startingmaterial will sufficiently increase. Therefore, in the hot workingprocess in the next process, the starting material can be made into abillet.

Note that, a thermometer (thermocouple) is disposed in the bloomreheating furnace, and it is possible to measure the in-furnacetemperature. Further, the holding time (minutes) in the bloom reheatingfurnace can be determined based on the time point at which the startingmaterial is charged into the bloom reheating furnace and the time pointat which the starting material is extracted from the bloom reheatingfurnace.

[(22) Hot Working Process]

In the hot working process, the starting material that was heated in thestarting material heating process is subjected to hot rolling to producea billet. Specifically, the heated starting material is subjected to hotrolling using a blooming mill to thereby produce a billet. After hotrolling by the blooming mill, as necessary, the starting material may besubjected to further hot rolling using a continuous mill arrangeddownstream of the blooming mill to produce a billet. The total reductionof area in the blooming process is not particularly limited, and forexample is 20 to 70%. The billet produced in the hot working process iscooled to normal temperature before the steel material productionprocess.

[(3) Steel Material Production Process]

In the steel material production process, the billet produced in theblooming process is subjected to hot working to produce a steelmaterial. The steel material production process includes the followingprocesses.

-   -   (31) Steel material heating process    -   (32) Hot working process

Hereunder, each process is described in detail.

[(31) Steel Material Heating Process]

In the steel material heating process, the billet produced in theblooming process is charged into a continuous heating furnace andheated. The heating furnace may be a rotary hearth heating furnace ormay be a walking beam heating furnace. In the following description, theuse of a rotary hearth heating furnace is described as one example of acontinuous heating furnace.

FIG. 6 is a schematic diagram (plan view) illustrating a rotary hearthheating furnace that is one example of a continuous heating furnace.Referring to FIG. 6 , a heating furnace 10 includes a furnace main body13 having a charging port 11 and an extraction port 12. A billet B1which is the object to be heated is charged into the heating furnace 10from the charging port 11. In FIG. 6 , the billet B1 is heated whilemoving through the inside of the heating furnace. In FIG. 6 , the billetB1 that was charged into the heating furnace 10 from the charging port11 moves in the clockwise direction. When the billet B1 which has beenheated while moving arrives at the extraction port 12, the billet B1 isextracted to outside from the extraction port 12.

The furnace main body 13 is divided into a preheating zone Z1, a heatingzone Z2, and a holding zone Z3 in that order in the direction from thecharging port 11 toward the extraction port 12. The preheating zone Z1is a zone that has the charging port 11. The preheating zone Z1 is thezone in which the in-furnace temperature is lowest among the three zones(preheating zone Z1, heating zone Z2 and holding zone Z3). The heatingzone Z2 is a zone arranged between the preheating zone Z1 and theholding zone Z3. The holding zone Z3 is a zone that follows the heatingzone Z2, and has the extraction port 12 at the rear end thereof. Theheating zone Z2 and the holding zone Z3 are maintained at approximatelythe same temperature. Specifically, although the temperature in theholding zone Z3 is somewhat higher than the temperature in the heatingzone Z2, the temperature difference between the holding zone Z3 and theheating zone Z2 is 20° C. or less. One or a plurality of burners isprovided in each of the zones. In each zone, the temperature is adjustedby means of the burner(s).

In the present embodiment the in-furnace temperature and the residencetime in the preheating zone Z1, the heating zone Z2, and the holdingzone Z3 are as follows.

[Preheating Zone Z1]

The in-furnace temperature and the residence time in the preheating zoneZ1 are as follows.

In-furnace temperature: a temperature from 1000 to less than 1275° C.,and which is a temperature that is lower than an in-furnace temperatureT in the heating zone Z2 and the holding zone Z3

Residence time: 100 minutes or more

In the preheating zone Z1, the in-furnace temperature is 1000 to lessthan 1275° C., and is set to a lower temperature than an in-furnacetemperature T (° C.) in the heating zone Z2 and the holding zone Z3. Inaddition, the residence time of the billet in the preheating zone Z1 isset to 100 minutes or more. The preheating zone Z1 mainly fulfills arole of increasing the temperature of the billet that is at normaltemperature. Preferably, the residence time in the preheating zone Z1 isset to 120 minutes or more, and more preferably is set to 130 minutes ormore.

[Heating Zone Z2 and Holding Zone Z3]

The conditions in the heating zone Z2 and the holding zone Z3 are asfollows.

In-furnace temperature T: a temperature from 1225 to 1275° C., and whichis a temperature that is higher than the in-furnace temperature in thepreheating zone Z1

Total residence time t: time that satisfies Formula (A)

These conditions are described hereunder.

(Regarding In-Furnace Temperature T)

With regard to the heating zone Z2 and the holding zone Z3, thein-furnace temperature T in the heating zone Z2 and the holding zone Z3is set in the range of 1225 to 1275° C., and is set to a temperaturethat is higher than the in-furnace temperature in the preheating zoneZ1. If the in-furnace temperature Tin the heating zone Z2 and theholding zone Z3 is less than 1225° C., the Cr concentrationdistribution, the Mo concentration distribution, and the Cuconcentration distribution within the segregation region SE will not beuniform, and variations will occur. Consequently, in the producedmartensitic stainless steel material, the degree of Cr segregation ΔCr,the degree of Mo segregation ΔMo, and the degree of Cu segregation ΔCuwill not satisfy Formula (4). On the other hand, if the in-furnacetemperature T in the heating zone Z2 and the holding zone Z3 is morethan 1275° C., δ-ferrite will be formed in the steel material having theaforementioned chemical composition. The δ-ferrite will decrease the hotworkability of the steel material. Accordingly, the in-furnacetemperature Tin the heating zone Z2 and the holding zone Z3 is to bewithin the range of 1225 to 1275° C.

(Regarding Total Residence Time t)

Let the total residence time in the heating zone Z2 and the holding zoneZ3 be defined as t (minute). The term “total residence time t” means thetime (minutes) from when the billet produced in the blooming processenters the heating zone Z2 until the billet is discharged to outsidefrom the extraction port 12. The in-furnace temperature T and the totalresidence time tin the heating zone Z2 and the holding zone Z3 are setso as to satisfy the following Formula (A):

B≤(t/60)^(0.5)×(T+273)  (A)

where, when the yield strength is 110 ksi grade (758 to less than 862MPa), B in Formula (A) is 2900, and when the yield strength is 862 MPaor more, B in Formula (A) is 3900.

In Formula (A), the total residence time t (minutes) of the billet inthe heating zone Z2 and the holding zone Z3 is substituted for “t”.Further, the in-furnace temperature T (° C.) in the heating zone Z2 andthe holding zone Z3 is substituted for “T”. Note that, an arithmeticaverage value of the in-furnace temperature (° C.) in the heating zoneZ2 obtained with a thermometer and the in-furnace temperature (° C.) inthe holding zone Z3 obtained with a thermometer is adopted as thein-furnace temperature T (° C.) in the heating zone Z2 and the holdingzone Z3.

Let FA be defined as FA=(t/60)^(0.5)×(T+273). FIG. 7A is a viewillustrating the relation between FA and a total degree of segregationΔF (=ΔCr+ΔMo+ΔCu) of Cr, Mo, and Cu in a microscopic segregation region(line segment LS) in a case where the yield strength of the steelmaterial is made 110 ksi grade (758 to less than 862 MPa). FIG. 7B is aview illustrating the relation between FA and the total degree ofsegregation ΔF in a case where the yield strength of the steel materialis made 125 ksi or more (862 MPa or more).

[Case where Yield Strength of Steel Material is Made 110 Ksi Grade]

Referring to FIG. 7A, in a case where the yield strength of the steelmaterial is made 110 ksi grade, if FA is less than 2900, the billet isnot sufficiently held in a temperature range of 1225° C. or more. Inthis case, at least one kind among variations in the Cr concentrationdistribution, variations in the Mo concentration distribution, andvariations in the Cu concentration distribution in the segregationregion SE in the billet cannot be sufficiently reduced. Therefore, asillustrated in FIG. 7A, in the produced martensitic stainless steelmaterial, the total degree of segregation ΔF is more than 0.70.

On the other hand, if FA is 2900 or more, the billet is sufficientlyheld in a temperature range of 1225° C. or more. In this case, in thesegregation region SE in the billet, variations in the Cr concentrationdistribution are sufficiently reduced, variations in the Moconcentration distribution are sufficiently reduced, and variations inthe Cu concentration distribution are sufficiently reduced. As a result,as illustrated in FIG. 7A, in comparison to when FA is less than 2900,the total degree of segregation ΔF in the produced martensitic stainlesssteel material markedly decreases, and becomes 0.70 or less. That is,variations in the Cr concentration, the Mo concentration, and the Cuconcentration in the segregation region SE can be markedly suppressed.

A preferable lower limit of FA in a case where the yield strength of thesteel material is made 110 ksi grade is 3000, more preferably is 3100,further preferably is 3150, further preferably is 3200, and furtherpreferably is 3250. An upper limit of FA is not particularly limited.However, taking into consideration the productivity during normalindustrial production, the total residence time t is preferably 600minutes or less. Accordingly, the upper limit of FA is, for example,4890.

Note that, a preferable lower limit of the total residence time t(minutes) in the heating zone Z2 and the holding zone Z3 in a case wherethe yield strength of the steel material is made 110 ksi grade is 220minutes, more preferably is 230 minutes, further preferably is 240minutes, and further preferably is 250 minutes.

In a case where the yield strength of the steel material is made 110 ksigrade, in the steel material heating process, the billet is heated usinga continuous heating furnace so that, in particular, FA is 2900 or morein the temperature range of 1225 to 1275° C. in the heating zone Z2 andthe holding zone Z3. Taking into consideration the residence time in thepreheating zone Z1, in the present embodiment a preferable furnace timeof the billet in the heating furnace is 320 minutes or more, and furtherpreferably is 330 minutes or more.

[Case where Yield Strength of Steel Material is Made 125 Ksi or More]

Referring to FIG. 7B, in a case where the yield strength of the steelmaterial is made 125 ksi or more, if FA is less than 3900, the billet isnot sufficiently held in a temperature range of 1225° C. or more. Inthis case, at least one kind among variations in the Cr concentrationdistribution, variations in the Mo concentration distribution, andvariations in the Cu concentration distribution in the segregationregion SE in the billet cannot be sufficiently reduced. Therefore, asillustrated in FIG. 7B, in the produced martensitic stainless steelmaterial, the total degree of segregation ΔF is more than 0.50.

On the other hand, if FA is 3900 or more, the billet is sufficientlyheld in the temperature range of 1225° C. or more. In this case, in thesegregation region SE in the billet, variations in the Cr concentrationdistribution are sufficiently reduced, variations in the Moconcentration distribution are sufficiently reduced, and variations inthe Cu concentration distribution are sufficiently reduced. As a result,as illustrated in FIG. 7B, in comparison to when FA is less than 3900,the total degree of segregation ΔF in the produced martensitic stainlesssteel material markedly decreases, and becomes 0.50 or less. That is,variations in the Cr concentration, the Mo concentration, and the Cuconcentration in the segregation region SE can be markedly suppressed.

A preferable lower limit of FA in a case where the yield strength of thesteel material is made 125 ksi or more is 3950, more preferably is 3980,and further preferably is 4000. An upper limit of FA is not particularlylimited. However, taking into consideration the productivity duringnormal industrial production, the total residence time t is preferably600 minutes or less. Accordingly, the upper limit of FA is, for example,4890.

Note that, a preferable lower limit of the total residence time t(minutes) in the heating zone Z2 and the holding zone Z3 in a case wherethe yield strength of the steel material is made 125 ksi or more is 350minutes, more preferably is 380 minutes, and further preferably is 400minutes.

In a case where the yield strength of the steel material is made 125 ksior more, in the steel material heating process, the billet is heatedusing a continuous heating furnace so that, in particular, FA is 3900 ormore in the temperature range of 1225 to 1275° C. in the heating zone Z2and the holding zone Z3. Taking into consideration the residence time inthe preheating zone Z1, in the present embodiment a preferable furnacetime of the billet in the heating furnace is 450 minutes or more, andfurther preferably is 500 minutes or more.

Note that, a thermometer (thermocouple) is arranged in each of thepreheating zone Z1, the heating zone Z2, and the holding zone Z3, andthus the in-furnace temperature in the respective zones can be measured.An arithmetic average value of the in-furnace temperature (° C.) in theheating zone Z2 obtained with a thermometer and the in-furnacetemperature (° C.) in the holding zone Z3 obtained with a thermometer isdefined as the in-furnace temperature T (° C.) in the heating zone Z2and the holding zone Z3. Further, the residence time of the billet ineach zone (preheating zone Z1, heating zone Z2, and holding zone Z3) canbe determined based on the order and feeding speed of the billetscharged into the heating furnace.

In the above description, a rotary hearth heating furnace has beendescribed as the heating furnace. However, the structure of a walkingbeam heating furnace is the same as the structure of a rotary hearthheating furnace. Specifically, a walking beam heating furnace includes amain body that has a charging port and an extraction port. The main bodyis divided into a preheating zone, a heating zone, and a holding zone inthat order in the direction from the charging port toward the extractionport. Accordingly, in a walking beam heating furnace also, theconditions of the heating process are as described above.

In FIG. 6 , the preheating zone Z1, the heating zone Z2, and the holdingzone Z3 are divided equally inside the furnace main body 13. However,the preheating zone Z1, the heating zone Z2, and the holding zone Z3 donot have to be divided equally.

In the production process of the present embodiment, an important pointis that heating for a long time period is not performed with respect tothe as-solidified starting material (bloom or billet), and instead thebillet subjected to hot working by the blooming process is subjected toheating for a long time period. The microstructure of the as-solidifiedstarting material includes dendrite (a tree-like structure). Dendriteinhibits diffusion of Cr, Mo, and Cu during heating. By preforming hotrolling on the starting material in the blooming process, dendrite isphysically or mechanically destroyed. Therefore, in comparison to themicrostructure of the starting material in the starting materialpreparation process, almost no dendritic structure is present in themicrostructure of the billet produced in the blooming process, and themicrostructure of the billet is a fine microstructure. By subjectingsuch a billet in which the amount of dendritic structure is small toheating under the aforementioned conditions, Cr, Mo, and Cu within thebillet can be adequately diffused. As a result, in the producedmartensitic stainless steel material, the degree of Cr segregation ΔCrdefined by Formula (1), the degree of Mo segregation ΔMo defined byFormula (2), and the degree of Cu segregation ΔCu defined by Formula (3)satisfy Formula (4).

[(32) Hot Working Process]

In the hot working process, the billet heated under the aforementionedconditions by the heating process is subjected to hot working. If theend product is a seamless steel pipe, the heated billet is subjected tohot working to produce a hollow shell (seamless steel pipe). Forexample, hot rolling by the Mannesmann-mandrel process is performed asthe hot working to produce a hollow shell. In this case, the billet issubjected to piercing-rolling by a piercing machine. When performingpiercing-rolling, although not particularly limited, the piercing ratiois, for example, 1.0 to 4.0. The billet after piercing-rolling issubjected to elongating and rolling using a mandrel mill. In addition,as needed, the billet after elongating and rolling is subjected todiameter adjusting rolling using a reducer or a sizing mill. A hollowshell is produced by the above process. Although not particularlylimited, the cumulative reduction of area in the hot working process is,for example, 20 to 70%.

If the end product is a round steel bar, for example, the heated billetis subjected to hot forging to produce a round steel bar.

[(4) Heat Treatment Process]

The heat treatment process includes the following processes.

-   -   (41) Quenching process    -   (42) Tempering process

Each process is described hereunder.

[(41) Quenching Process]

In the heat treatment process, first, the steel material (hollow shellor round steel bar) produced in the hot working process is subjected toquenching (quenching process). The quenching is performed by awell-known method. Specifically, the steel material after the hotworking process is charged into a heat treatment furnace and held at aquenching temperature. The quenching temperature is equal to or higherthan the A_(C3) transformation point and, for example, is 900 to 1000°C. After being held at the quenching temperature, the steel material israpidly cooled (quenched). Although not particularly limited, theholding time at the quenching temperature is for example, 10 to 60minutes. The quenching method is, for example, water cooling or oilcooling. The quenching method is not particularly limited. For example,the hollow shell may be rapidly cooled by immersing the hollow shell ina water bath or an oil bath, or the hollow shell may be rapidly cooledby pouring or jetting cooling water onto the outer surface and/or innersurface of the hollow shell by shower cooling or mist cooling.

In a case where the martensitic stainless steel material is a seamlesssteel pipe, after the hot working process, quenching (direct quenching)may be performed immediately after the hot working, without cooling thehollow shell to normal temperature. Further, quenching may be performedafter the hollow shell after hot working has been held at the quenchingtemperature after being charged into a supplementary heating furnacebefore the temperature of the hollow shell decreased after the hotworking.

[(42) Tempering Process]

The hollow shell after quenching is also subjected to a temperingprocess. In the tempering process, the yield strength of the steelmaterial is adjusted. For the martensitic stainless steel material ofthe present embodiment, the tempering temperature is set in the range of550° C. to the A_(C1) transformation point.

In a case where the yield strength of the steel material is to be made110 ksi grade (758 to less than 862 MPa), a preferable lower limit ofthe tempering temperature is 610° C., and more preferably is 620° C. Apreferable upper limit of the tempering temperature is 640° C., and morepreferably is 635° C.

In a case where the yield strength of the steel material is to be made125 ksi or more (862 MPa or more), a preferable lower limit of thetempering temperature is 575° C., and more preferably is 580° C. Apreferable upper limit of the tempering temperature is less than 610°C., and more preferably is 605° C.

Although not particularly limited, the holding time at the temperingtemperature is, for example, 20 to 60 minutes. A preferable upper limitof the holding time is 50 minutes, and more preferably is 45 minutes. Byappropriately adjusting the tempering temperature according to thechemical composition, the yield strength of the martensitic stainlesssteel material can be adjusted. Specifically, the tempering conditionsare adjusted so that the yield strength of the martensitic stainlesssteel material becomes 110 ksi or more (758 MPa or more).

The martensitic stainless steel material of the present embodiment canbe produced by the processes described above.

Example 1

The advantageous effect of one aspect of the steel material of thepresent embodiment will be described more specifically by way ofexamples. The conditions adopted in the following examples are oneexample of conditions employed for confirming the workability andadvantageous effects of the steel material of the present embodiment.Accordingly, the steel material of the present embodiment is not limitedto this one example of the conditions.

In Example 1, steel materials having a yield strength of 110 ksi grade(758 to less than 862 MPa) were produced, and various evaluation testswere performed. The details are described hereunder.

[Production of Steel Material]

[Starting Material Preparation Process]

Molten steels having the chemical compositions shown in Table 1 wereproduced.

TABLE 1 Test Chemical Composition Values (mass %; balance: Fe andimpurities) No. C Si Mn P S Ni Cr Mo Al V N 1 0.008 0.23 0.44 0.0150.0007 5.80 13.20 2.34 0.040 0.04 0.0054 2 0.027 0.33 0.39 0.013 0.00045.21 13.02 2.15 0.027 0.05 0.0038 3 0.012 0.23 0.38 0.016 0.0009 5.3013.40 2.57 0.037 0.03 0.0059 4 0.012 0.30 0.40 0.014 0.0005 5.99 10.102.51 0.037 0.04 0.0065 5 0.008 0.37 0.48 0.016 0.0005 5.87 13.30 2.340.037 0.04 0.0065 6 0.010 0.24 0.40 0.011 0.0009 5.33 13.00 2.00 0.0370.04 0.0065 7 0.009 0.31 0.41 0.017 0.0006 5.67 12.70 2.89 0.037 0.040.0065 8 0.008 0.27 0.46 0.016 0.0008 5.60 13.30 2.68 0.037 0.04 0.00659 0.009 0.34 0.43 0.013 0.0007 6.06 11.30 2.32 0.037 0.04 0.0065 100.011 0.36 0.34 0.011 0.0008 5.56 12.70 2.50 0.029 0.05 0.0062 11 0.0120.26 0.40 0.011 0.0008 5.43 13.10 2.36 0.027 0.03 0.0051 12 0.029 0.250.41 0.013 0.0006 5.38 12.89 2.00 0.026 0.05 0.0047 13 0.026 0.27 0.400.015 0.0006 5.40 12.86 2.02 0.031 0.05 0.0048 14 0.009 0.25 0.39 0.0130.0007 6.15 12.50 2.54 0.029 0.04 0.0055 15 0.011 0.32 0.30 0.010 0.00076.00 12.50 2.44 0.037 0.05 0.0051 16 0.009 0.25 0.30 0.011 0.0006 5.6013.20 2.70 0.036 0.04 0.0063 17 0.010 0.26 0.38 0.013 0.0008 5.74 11.302.11 0.030 0.06 0.0071 18 0.009 0.31 0.37 0.017 0.0005 5.89 11.90 2.350.031 0.04 0.0063 19 0.011 0.35 0.41 0.012 0.0008 5.49 12.40 2.31 0.0380.06 0.0055 20 0.009 0.30 0.35 0.010 0.0008 5.53 12.30 2.64 0.041 0.060.0043 21 0.008 0.36 0.40 0.013 0.0010 5.74 12.30 2.40 0.025 0.06 0.006422 0.008 0.31 0.37 0.017 0.0006 5.28 11.40 2.57 0.034 0.06 0.0035 230.009 0.35 0.44 0.014 0.0009 6.11 11.60 2.48 0.031 0.06 0.0050 24 0.0120.37 0.42 0.014 0.0009 5.86 9.50 2.43 0.041 0.03 0.0053 25 0.010 0.300.42 0.015 0.0006 6.07 15.50 2.32 0.029 0.04 0.0048 26 0.009 0.32 0.410.011 0.0005 5.67 12.70 1.41 0.041 0.06 0.0044 27 0.008 0.33 0.38 0.0110.0009 5.72 13.10 3.12 0.037 0.07 0.0048 28 0.009 0.30 0.40 0.013 0.00085.88 12.00 2.44 0.039 0.04 0.0055 29 0.009 0.27 0.40 0.016 0.0009 5.5713.20 2.47 0.030 0.05 0.0045 30 0.009 0.21 0.48 0.014 0.0010 5.57 12.281.93 0.029 0.05 0.0065 31 0.009 0.25 0.40 0.015 0.0010 6.01 13.40 2.260.044 0.05 0.0066 32 0.008 0.28 0.36 0.011 0.0008 5.23 11.20 2.70 0.0360.03 0.0058 33 0.012 0.38 0.31 0.014 0.0011 5.54 12.80 2.68 0.031 0.070.0049 34 0.011 0.31 0.44 0.016 0.0011 6.05 11.20 2.68 0.041 0.03 0.004035 0.012 0.32 0.47 0.013 0.0011 6.07 11.70 2.64 0.041 0.03 0.0038 360.011 0.37 0.43 0.014 0.0011 5.29 11.10 2.57 0.042 0.06 0.0051 37 0.0110.37 0.46 0.017 0.0009 6.12 11.20 2.52 0.026 0.04 0.0049 38 0.009 0.380.45 0.017 0.0008 6.00 12.10 2.68 0.037 0.04 0.0065 39 0.010 0.24 0.400.015 0.0010 6.05 12.50 2.60 0.035 0.05 0.0042 Test Chemical CompositionValues (mass %; balance: Fe and impurities) No. Ti Cu Co B Ca Mg REM NbW 1 0.110 1.92 0.34 — — — — — — 2 0.060 2.10 0.15 — — — — — — 3 0.0901.96 0.13 — — — — — — 4 0.120 1.86 0.33 — — — — — — 5 0.100 2.71 0.36 —— — — — — 6 0.090 2.89 0.23 — — — — — — 7 0.100 2.65 0.18 — — — — — — 80.070 1.87 0.33 — — — — — — 9 0.080 3.24 0.28 — — — — — — 10 0.080 2.910.18 0.0001 — — — — — 11 0.050 2.66 0.28 — 0.0033 — — — — 12 0.066 2.110.18 — — 0.0043 — — — 13 0.068 2.08 0.21 — — — 0.0045 — — 14 0.050 2.920.10 — — — — 0.02 — 15 0.060 3.06 0.29 — — — — — 0.12 16 0.090 2.04 0.22— — — — 0.03 0.15 17 0.080 2.07 0.20 0.0003 — — — 0.03 0.12 18 0.0501.96 0.38 0.0001 — — — 0.04 0.14 19 0.080 2.84 0.20 0.0001 0.0030 — —0.09 0.11 20 0.100 2.88 0.25 0.0002 — — 0.0010 0.06 0.08 21 0.090 2.900.12 0.0004 — 0.0010 0.0015 0.02 0.19 22 0.110 2.89 0.25 0.0001 — 0.00170.0017 0.04 0.14 23 0.090 2.48 0.31 0.0004 0.0027 0.0016 0.0014 0.020.18 24 0.100 2.88 0.13 — — — — — — 25 0.050 2.45 0.24 — — — — — — 260.070 2.03 0.22 — — — — — — 27 0.090 2.28 0.36 — — — — — — 28 0.080 0.920.20 — — — — — — 29 0.130 3.81 0.31 — — — — — — 30 0.091 2.01 0.12 — — —— — — 31 0.090 2.09 0.24 — — — — — — 32 0.070 1.98 0.23 — — — — 0.02 —33 0.120 2.64 0.29 — — — — — 0.05 34 0.080 2.99 0.26 — — — — 0.02 0.1435 0.050 2.42 0.17 0.0001 — — — — — 36 0.120 1.89 0.19 — 0.0015 — — — —37 0.080 2.89 0.18 0.0004 0.0027 — — — — 38 0.080 2.40 0.25 0.00010.0009 0.0005 0.0005 0.04 0.11 39 0.110 2.47 0.17 — — — — — —

In Table 1, the “-” symbol means that the content of the correspondingelement was less than the detection limit. Specifically, for example,with regard to Test Number 1 in Table 1, the “-” symbol means that thecontent of Nb was 0% (0.00%) when rounded off to the second decimalplace, and that the content of W was 0% (0.00%) when rounded off to thesecond decimal place.

Each of the produced molten steels was used to produce a bloom bycontinuous casting.

[Blooming Process]

Next, in a blooming process, each bloom was subjected to hot rolling toproduce a cylindrical billet (round billet) having a diameter of 310 mm.Specifically, first, the bloom was heated in a bloom reheating furnace.The in-furnace temperature (° C.) of the bloom reheating furnace and theholding time (minutes) in the bloom reheating furnace for each testnumber were as shown in Table 2.

TABLE 2 Steel Material Production Process Total Blooming ProcessIn-furnace Residence In-furnace Holding Temperature Time t TemperatureTime in Residence T (° C.) in (min) in Furnace in Bloom Bloom In-furnaceTime in Heating Heating Time in Reheating Reheating TemperaturePreheating Zone and Zone and Heating Test Furnace Furnace PreheatingZone Holding Holding Furnace No. (° C.) (min) Zone (2° C.) (min) ZoneZone FA (min) ΔCr 1 1250 233 1100 157 1250 401 3937 558 0.07 2 1270 2021100 152 1250 378 3823 530 0.05 3 1270 220 1050 148 1275 260 3222 4080.06 4 1270 280 1060 180 1250 354 3699 534 0.05 5 1260 269 1090 153 1250379 3828 532 0.07 6 1250 269 1100 171 1250 388 3873 559 0.06 7 1260 2181130 173 1250 317 3501 490 0.07 8 1260 240 1140 139 1250 229 2975 3680.06 9 1270 380 1080 186 1250 446 4152 632 0.05 10 1260 255 1130 1721250 546 4594 718 0.06 11 1270 256 1060 160 1250 417 4015 577 0.06 121250 320 1100 147 1250 360 3731 507 0.06 13 1250 209 1100 150 1250 3623741 512 0.06 14 1260 271 1140 176 1250 355 3705 531 0.05 15 1270 2601150 181 1250 502 4405 683 0.06 16 1250 239 1120 165 1250 307 3445 4720.06 17 1260 238 1060 160 1250 225 2949 385 0.04 18 1260 222 1130 1651250 356 3710 521 0.06 19 1260 301 1100 202 1250 546 4594 748 0.06 201250 235 1100 178 1250 502 4405 680 0.06 21 1270 245 1100 180 1250 4614222 641 0.05 22 1270 254 1130 186 1250 445 4148 631 0.05 23 1250 2951060 168 1225 305 3377 473 0.04 24 1250 238 1100 170 1250 458 4208 6280.05 25 1270 268 1070 160 1250 263 3189 423 0.14 26 1260 279 1120 1421250 257 3152 399 0.07 27 1260 283 1060 166 1250 381 3838 547 0.10 281250 315 1120 161 1250 374 3802 535 0.07 29 1260 340 1110 181 1250 3983923 579 0.11 30 1250 206 1090 136 1250 173 2586 309 0.08 31 1250 2291070 140 1250 109 2053 249 0.10 32 1270 284 1110 160 1250 188 2696 3480.09 33 1250 280 1130 151 1250 176 2608 327 0.10 34 1260 305 1150 1551250 187 2689 342 0.11 35 1250 223 1070 154 1250 139 2318 293 0.09 361260 237 1120 154 1250 155 2448 309 0.11 37 1250 237 1090 151 1250 1592479 310 0.09 38 1250 282 1060 146 1250 145 2368 291 0.09 39 1260 2671080 155 1225 222 2881 377 0.11 Tempering Process Holding Yield TestTemperature Time Strength No. ΔMo ΔCu ΔF (° C.) (min) (MPa) SSCResistance 1 0.25 0.19 0.51 640 20 818 P 2 0.28 0.14 0.47 620 43 851 P 30.27 0.14 0.47 639 37 794 P 4 0.42 0.1 0.57 637 32 832 P 5 0.33 0.140.54 636 26 818 P 6 0.25 0.22 0.53 636 41 788 P 7 0.31 0.24 0.62 639 21805 P 8 0.32 0.15 0.53 634 34 790 P 9 0.27 0.17 0.49 639 38 825 P 10 0.20.19 0.45 632 23 798 P 11 0.18 0.18 0.42 639 34 778 P 12 0.29 0.19 0.54620 40 853 P 13 0.29 0.19 0.54 620 40 857 P 14 0.3 0.14 0.49 632 21 823P 15 0.28 0.14 0.48 637 23 801 P 16 0.25 0.16 0.47 637 26 823 P 17 0.290.08 0.41 632 34 783 P 18 0.33 0.09 0.48 633 26 803 P 19 0.25 0.14 0.45639 43 785 P 20 0.26 0.14 0.46 633 43 826 P 21 0.25 0.14 0.44 639 37 814P 22 0.29 0.13 0.47 632 36 815 P 23 0.29 0.11 0.44 631 33 839 P 24 0.260.23 0.54 632 23 788 F 25 0.27 0.39 0.80 630 39 858 F 26 0.23 0.20 0.50638 24 762 F 27 0.39 0.33 0.82 639 32 835 F 28 0.43 0.10 0.60 635 28 798F 29 0.26 0.39 0.76 636 41 857 F 30 0.36 0.30 0.74 636 22 833 F 31 0.450.22 0.77 639 28 834 F 32 0.48 0.24 0.81 633 23 817 F 33 0.27 0.42 0.79633 35 840 F 34 0.28 0.45 0.84 638 27 831 F 35 0.35 0.41 0.85 637 32 781F 36 0.42 0.32 0.85 635 31 787 F 37 0.24 0.46 0.79 637 26 807 F 38 0.320.37 0.78 635 24 793 F 39 0.40 0.25 0.76 632 25 826 F

After the bloom was heated in the bloom reheating furnace, the heatedbloom was subjected to hot rolling using a blooming mill to produce around billet having a diameter of 310 mm.

[Steel Material Production Process]

The round billet of each test number was subjected to a steel materialheating process. Specifically, the round billet of each test number wasloaded into a rotary hearth heating furnace. The in-furnace temperature(° C.) of the preheating zone, the residence time (minutes) in thepreheating zone, the in-furnace temperature T (° C.) in the heating zoneand the holding zone, and the total residence time t (minutes) in theheating zone and the holding zone in the heating furnace were as shownin Table 2. Further, FA=(t/60)^(0.5)×(T+273) was as shown in Table 2.Note that, an arithmetic average value of an in-furnace temperature (°C.) in the heating zone Z2 obtained with a thermometer and an in-furnacetemperature (° C.) in the holding zone Z3 obtained with a thermometerwas adopted as the in-furnace temperature T (° C.) in the heating zoneand the holding zone.

Each of the round billets heated by the steel material heating processwas subjected to a hot working process. Specifically, each round billetwas subjected to hot rolling by the Mannesmann-mandrel process tothereby produce a hollow shell (seamless steel pipe) of each testnumber. At such time, the piercing ratio was within the range of 1.0 to4.0, and the cumulative reduction of area in the hot working process waswithin the range of 20 to 70%.

[Heat Treatment Process]

Each of the produced hollow shells was subjected to a heat treatmentprocess (quenching process and tempering process). In the quenchingprocess, the quenching temperature was set to 910° C., and the holdingtime at the quenching temperature was set to 15 minutes. In thetempering process, the tempering temperature (° C.) was set as shown inTable 2, and the holding time (minutes) at the tempering temperature wasset as shown in Table 2. The yield strength was adjusted to 110 ksigrade (758 to less than 862 MPa) by the heat treatment process.Martensitic stainless steel materials (seamless steel pipes) wereproduced by the above production process.

[Evaluation Test]

The seamless steel pipe of each test number was subjected to thefollowing evaluation tests.

-   -   (1) Microstructure observation test    -   (2) Cr concentration, Mo concentration, and Cu concentration        measurement test    -   (3) Tensile test    -   (4) SSC resistance evaluation test

[(1) Microstructure Observation Test]

The volume ratio of martensite of the seamless steel pipe of each testnumber was measured by the following method. Specifically, the volumeratio (%) of retained austenite was determined, and the determined valuewas subtracted from 100.0% to determine the martensite volume ratio.

The volume ratio of retained austenite was determined by an X-raydiffraction method. Specifically, a test specimen was taken from thecenter portion of the wall thickness of the seamless steel pipe. Thesize of the test specimen was 15 mm×15 mm×a thickness of 2 mm. Thethickness direction of the test specimen was the wall thicknessdirection of the seamless steel pipe. Using the obtained test specimen,the X-ray diffraction intensity of each of the (200) plane of α phase,the (211) plane of α phase, the (200) plane of γ phase, the (220) planeof γ phase, and the (311) plane of γ phase was measured, and theintegrated intensity of each plane was calculated. In the measurement ofthe X-ray diffraction intensity, the target of the X-ray diffractionapparatus was Mo (MoKα ray), and the output was set to 50 kV and 40 mA.After calculation, the volume ratio Vγ (%) of retained austenite wascalculated using Formula (I) for combinations (2×3=6 pairs) of eachplane of the α phase and each plane of the γ phase. Then, an averagevalue of the volume ratios Vγ of retained austenite of the six pairs wasdefined as the volume ratio (%) of retained austenite.

Vγ=100/{1+(Iα×Rγ)/(Iγ×Rα)}  (I)

Where, Iα is an integrated intensity of α phase. Rα is acrystallographic theoretical calculation value of α phase. Iγ is anintegrated intensity of γ phase. Rγ is a crystallographic theoreticalcalculation value of γ phase. Note that, Rα in the (200) plane of αphase was set to 15.9, Rα in the (211) plane of α phase was set to 29.2,Rγ in the (200) plane of γ phase was set to 35.5, Rγ in the (220) planeof γ phase was set to 20.8, and Rγ in the (311) plane of γ phase was setto 21.8. The volume ratio of retained austenite was obtained by roundingoff the second decimal place of the obtained numerical value.

The volume ratio (%) of retained austenite obtained by the X-raydiffraction method described above was used to obtain the volume ratio(%) of martensite in the microstructure of the seamless steel pipe bythe following Formula.

Volume ratio of martensite=100.0−volume ratio of retained austenite (%)

The measurement results showed that in each test number the volume ratioof martensite was 80.0% or more.

[(2) Cr Concentration, Mo Concentration, and Cu ConcentrationMeasurement Test]

The degree of Cr segregation ΔCr, the degree of Mo segregation ΔMo, andthe degree of Cu segregation ΔCu of each test number were determined bythe following method.

In a cross section including a rolling direction L and a wall thicknessdirection T of the seamless steel pipe, an arbitrary two points atpositions at a depth of 2 mm from the inner surface were defined as twocenter points P1. Two line segments of 1000 μm extending in the wallthickness direction T with each center point P1 as a center were definedas two line segments LS. On each line segment LS, point analysis usingenergy dispersive X-ray spectroscopy (EDS) was performed at measurementpositions at a pitch of 1 μm, and the Cr concentration (mass %), the Moconcentration (mass %), and the Cu concentration (mass %) at eachmeasurement position were determined. In the point analysis, theaccelerating voltage was set to 20 kV.

The following items were defined based on the measured Cr concentration,Mo concentration, and Cu concentration.

-   -   (A) An average value of all of the Cr concentrations determined        at all of the measurement positions on the two line segments LS        was defined as [Cr]_(ave).    -   (B) A sample standard deviation of all of the Cr concentrations        determined at all of the measurement positions on the two line        segments LS was defined as σ_(Cr).    -   (C) Based on the three sigma rule, among all of the Cr        concentrations determined at all of the measurement positions on        the two line segments LS, an average value of the Cr        concentrations included within a range of [Cr]_(ave)±3σ_(Cr) was        defined as [Cr*]_(ave).    -   (D) Among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) was defined as [Cr*]_(max).    -   (E) Among all of the Cr concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Cr concentrations included within a range of        [Cr]_(ave)±3σ_(Cr) was defined as [Cr*]_(min).    -   (F) An average value of all of the Mo concentrations determined        at all of the measurement positions on the two line segments LS        was defined as [Mo]_(ave).    -   (G) A sample standard deviation of all of the Mo concentrations        determined at all of the measurement positions on the two line        segments LS was defined as σ_(Mo).    -   (H) Based on the three sigma rule, among all of the Mo        concentrations determined at all of the measurement positions on        the two line segments LS, an average value of the Mo        concentrations included within a range of [Mo]_(ave)±3σ_(Mo) was        defined as [Mo*]_(ave).    -   (I) Among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) was defined as [Mo*]_(max).    -   (J) Among all of the Mo concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Mo concentrations included within a range of        [Mo]_(ave)±3σ_(Mo) was defined as [Mo*]_(min).    -   (K) An average value of all of the Cu concentrations determined        at all of the measurement positions on the two line segments LS        was defined as [Cu]_(ave).    -   (L) A sample standard deviation of all of the Cu concentrations        determined at all of the measurement positions on the two line        segments LS was defined as σ_(Cu).    -   (M) Based on the three sigma rule, among all of the Cu        concentrations determined at all of the measurement positions on        the two line segments LS, an average value of the Cu        concentrations included within a range of [Cu]_(ave)±3σ_(Cu) was        defined as [Cu*]_(ave).    -   (N) Among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, a maximum        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) was defined as [Cu*]_(max).    -   (O) Among all of the Cu concentrations determined at all of the        measurement positions on the two line segments LS, a minimum        value of the Cu concentrations included within a range of        [Cu]_(ave)±3σ_(Cu) was defined as [Cu*]_(min).

Based on the items determined in the above (A) to (O), a degree of Crsegregation ΔCr defined by Formula (1) was determined, a degree of Mosegregation ΔMo defined by Formula (2) was determined, and a degree ofCu segregation ΔCu defined by Formula (3) was determined.

ΔCr=([Cr*]_(max)−[Cr*]_(min))/[Cr*]_(ave)  (1)

ΔMo=([Mo*]_(max)−[Mo*]_(min))/[Mo*]_(ave)  (2)

ΔCu=([Cu*]_(max)−[Cu*]_(min))/[Cu*]_(ave)  (3)

Based on the obtained degree of Cr segregation ΔCr, degree of Mosegregation ΔMo, and degree of Cu segregation ΔCu, a total degree ofsegregation ΔF defined by the following formula was determined.

ΔF=ΔCr+ΔMo+ΔCu

The degree of Cr segregation ΔCr, the degree of Mo segregation ΔMo, thedegree of Cu segregation ΔCu, and ΔF are shown in Table 2.

[(3) Tensile Test]

The yield strength of the seamless steel pipe of each test number wasdetermined by the following method. A tensile test specimen was takenfrom the center portion of the wall thickness of the seamless steelpipe. The tensile test specimen was a round bar tensile test specimen inwhich the diameter of the parallel portion was 6.0 mm, and the length ofthe parallel portion was 40.0 mm. The longitudinal direction of theparallel portion of the round bar tensile test specimen was parallel tothe rolling direction (longitudinal direction) of the seamless steelpipe. A tensile test was conducted at 24° C. in conformity with ASTME8/E8M (2013) using the round bar tensile test specimen, and the 0.2%offset proof stress (MPa) was determined. The determined 0.2% offsetproof stress was defined as the yield strength (MPa). The obtained yieldstrength is shown in Table 2.

[(4) SSC Resistance Evaluation Test]

The seamless steel pipe of each test number was subjected to an SSCresistance evaluation test in accordance with NACE TM0177-2005 Method A.A round bar specimen was taken from the center portion of the wallthickness of the seamless steel pipe. The round bar specimen had a sizein which the diameter of the parallel portion was 6.35 mm, and thelength of the parallel portion was 25.4 mm. The longitudinal directionof the parallel portion of the round bar specimen was parallel to therolling direction (longitudinal direction) of the seamless steel pipe.

An aqueous solution containing 20 mass % of sodium chloride in which thepH was 4.0 was adopted as the test solution. A stress equivalent to 90%of the actual yield stress was applied to the round bar specimen. Thetest solution at 24° C. was poured into a test vessel so that the roundbar specimen to which the stress had been applied was immersed therein,and this was adopted as the test bath. After degassing the test bath, agaseous mixture consisting of H₂S at 0.10 bar and CO₂ at 0.90 bar wasblown into the test bath so that the test bath was saturated with H₂Sgas. The test bath in which the H₂S gas was saturated was held at 24° C.for 720 hours. After the test specimen had been held for 720 hours, thesurface of the test specimen was observed with a magnifying glass with amagnification of ×10 to check for the presence of cracking. If a placewhere cracking was suspected was found in the observation with themagnifying glass, a cross section at the place where cracking wassuspected was observed with an optical microscope with a magnificationof ×100 to confirm whether cracking was present.

If the result of confirming whether cracking was present was thatcracking was not confirmed even when observed with the magnifying glasswith a magnification of ×10 and the optical microscope with amagnification of ×100, the relevant seamless steel pipe was evaluated asbeing excellent in SSC resistance (described as “P” (Pass) in the column“SSC resistance” in Table 2). On the other hand, if cracking wasconfirmed when the surface of the test specimen was observed with themagnifying glass with a magnification of ×10 or the optical microscopewith a magnification of ×100, the relevant seamless steel pipe wasevaluated as having low SSC resistance (described as “F” (Fail) in thecolumn “SSC resistance” in Table 2).

[Evaluation Results]

Referring to Table 2, in Test Numbers 1 to 23, the content of eachelement in the chemical composition was within the range of the presentembodiment. In addition, in the heating process, the in-furnacetemperature and residence time in the preheating zone were appropriate,the in-furnace temperature T in the heating zone and the holding zonewas 1225 to 1275° C., and FA was 2900 or more. Therefore, the totaldegree of segregation ΔF was 0.70 or less, and the Cr concentrationdistribution, the Mo concentration distribution, and the Cuconcentration distribution in a microscopic segregation region in thesteel material were sufficiently uniform. As a result, the yieldstrength was 110 ksi grade (758 to less than 862 MPa), and excellent SSCresistance was obtained.

In Test Number 24, the content of Cr was too low. Therefore, the SSCresistance was low.

In Test Number 25, the content of Cr was too high. Therefore, the totaldegree of segregation ΔF was more than 0.70. As a result, the SSCresistance was low.

In Test Number 26, the content of Mo was too low. Therefore, the SSCresistance was low.

In Test Number 27, the content of Mo was too high. Therefore, the totaldegree of segregation ΔF was more than 0.70. As a result, the SSCresistance was low.

In Test Number 28, the content of Cu was too low. Therefore, the SSCresistance was low.

In Test Number 29, the content of Cu was too high. Therefore, the totaldegree of segregation ΔF was more than 0.70. As a result, the SSCresistance was low.

On the other hand, in Test Numbers 30 to 39, although the content ofeach element in the chemical composition was within the range of thepresent embodiment, FA was less than 2900 and Formula (A) was notsatisfied. Therefore, the total degree of segregation ΔF in these testnumbers was more than 0.70. As a result, in these test numbers the SSCresistance was low.

Example 2

Steel materials (seamless steel pipes) having a yield strength of 125ksi or more (862 MPa or more) were produced by the same productionmethod as the method used in Example 1. The produced steel materialswere subjected to the same evaluation tests as in Example 1.

[Production of Steel Material]

[Starting Material Preparation Process]

Molten steels having the chemical compositions shown in Table 3 wereproduced.

TABLE 3 Test Chemical Composition Values (mass %; balance: Fe andimpurities) No. C Si Mn P S Ni Cr Mo Al V N 1 0.008 0.38 0.30 0.0120.0011 6.03 12.40 2.33 0.034 0.04 0.0043 2 0.028 0.33 0.41 0.012 0.00055.89 13.00 2.50 0.031 0.06 0.0051 3 0.009 0.33 0.46 0.013 0.0005 5.7212.00 2.49 0.029 0.07 0.0066 4 0.010 0.27 0.35 0.010 0.0006 5.87 12.902.37 0.038 0.05 0.0062 5 0.010 0.30 0.41 0.011 0.0009 5.98 13.00 2.550.038 0.05 0.0044 6 0.008 0.28 0.45 0.014 0.0008 5.62 10.30 2.37 0.0370.04 0.0065 7 0.012 0.25 0.34 0.016 0.0007 5.95 13.30 2.11 0.037 0.040.0065 8 0.012 0.33 0.36 0.016 0.0005 5.70 13.20 2.01 0.037 0.04 0.00659 0.010 0.27 0.43 0.017 0.0005 5.45 12.20 2.66 0.037 0.04 0.0065 100.011 0.29 0.34 0.016 0.0005 5.30 11.10 2.52 0.037 0.04 0.0065 11 0.0100.28 0.32 0.013 0.0006 5.48 11.10 2.38 0.037 0.04 0.0065 12 0.012 0.360.35 0.011 0.0011 5.73 12.20 2.54 0.030 0.05 0.0037 13 0.011 0.26 0.490.017 0.0008 6.01 12.50 2.32 0.038 0.04 0.0045 14 0.029 0.29 0.37 0.0110.0005 6.08 12.70 2.47 0.034 0.05 0.0053 15 0.029 0.27 0.35 0.013 0.00056.05 12.80 2.48 0.034 0.04 0.0058 16 0.008 0.38 0.37 0.017 0.0007 6.0312.60 2.40 0.036 0.04 0.0062 17 0.010 0.24 0.42 0.011 0.0006 5.76 13.102.51 0.035 0.05 0.0036 18 0.012 0.22 0.37 0.016 0.0011 5.50 11.20 2.620.027 0.07 0.0050 19 0.009 0.27 0.40 0.012 0.0010 6.14 12.70 2.40 0.0340.04 0.0040 20 0.010 0.22 0.32 0.011 0.0009 5.62 11.70 2.53 0.033 0.040.0052 21 0.011 0.23 0.42 0.012 0.0009 5.94 12.80 2.32 0.035 0.03 0.003722 0.010 0.27 0.48 0.013 0.0005 5.83 11.60 2.40 0.034 0.04 0.0054 230.011 0.26 0.49 0.012 0.0007 5.22 11.50 2.68 0.029 0.06 0.0043 24 0.0120.37 0.47 0.011 0.0009 5.98 11.80 2.46 0.035 0.04 0.0036 25 0.011 0.370.49 0.012 0.0006 5.80 9.50 2.70 0.031 0.05 0.0042 26 0.010 0.24 0.490.011 0.0006 5.49 15.50 2.68 0.026 0.05 0.0033 27 0.012 0.37 0.32 0.0120.0006 5.62 11.70 1.41 0.025 0.07 0.0063 28 0.009 0.25 0.32 0.011 0.00065.71 13.40 3.12 0.032 0.04 0.0048 29 0.012 0.32 0.37 0.017 0.0009 5.6412.30 2.33 0.029 0.06 0.0060 30 0.012 0.32 0.47 0.017 0.0005 5.80 13.302.68 0.038 0.07 0.0064 31 0.009 0.21 0.48 0.014 0.0010 5.57 12.28 1.930.029 0.05 0.0065 32 0.010 0.33 0.38 0.012 0.0010 6.04 13.20 2.29 0.0300.05 0.0049 33 0.010 0.29 0.36 0.016 0.0005 5.39 13.30 2.60 0.025 0.050.0034 34 0.010 0.22 0.48 0.013 0.0011 5.78 11.20 2.56 0.033 0.06 0.005735 0.011 0.25 0.45 0.016 0.0007 6.13 12.50 2.38 0.034 0.05 0.0066 360.012 0.26 0.49 0.017 0.0005 6.06 12.30 2.70 0.029 0.05 0.0033 37 0.0080.25 0.40 0.016 0.0008 5.68 11.60 2.50 0.026 0.05 0.0053 38 0.012 0.280.31 0.014 0.0006 5.85 11.60 2.68 0.035 0.06 0.0055 39 0.008 0.26 0.480.015 0.0008 6.14 12.40 2.37 0.037 0.04 0.0065 40 0.009 0.26 0.40 0.0120.0009 5.84 12.80 2.50 0.033 0.05 0.0050 Test Chemical CompositionValues (mass %; balance: Fe and impurities) No. Ti Cu Co B Ca Mg REM NbW 1 0.110 1.85 0.19 — — — — — — 2 0.120 2.21 0.24 — — — — — — 3 0.1202.89 0.10 — — — — — — 4 0.100 2.61 0.20 — — — — — — 5 0.080 2.70 0.25 —— — — — — 6 0.050 2.10 0.22 — — — — — — 7 0.110 2.21 0.10 — — — — — — 80.090 1.99 0.17 — — — — — — 9 0.120 2.21 0.38 — — — — — — 10 0.050 1.610.12 — — — — — — 11 0.070 3.13 0.35 — — — — — — 12 0.060 2.48 0.210.0004 — — — — — 13 0.060 2.80 0.19 — 0.0030 — — — — 14 0.110 2.18 0.32— — 0.0037 — — — 15 0.110 2.23 0.28 — — — 0.0035 — — 16 0.130 2.92 0.35— — — — 0.11 — 17 0.060 1.81 0.21 — — — — — 0.07 18 0.100 2.94 0.26 — —— — 0.04 0.07 19 0.110 2.80 0.34 0.0003 — — — 0.04 0.09 20 0.090 1.810.13 0.0003 0.0033 — — 0.04 0.12 21 0.070 2.85 0.27 0.0002 — — 0.00120.05 0.08 22 0.050 2.40 0.29 0.0003 — 0.0010 0.0012 0.11 0.12 23 0.0602.83 0.13 0.0004 — 0.0017 0.0014 0.01 0.05 24 0.100 2.99 0.24 0.00050.0029 0.0015 0.0010 0.06 0.05 25 0.090 2.70 0.35 — — — — — — 26 0.0602.43 0.17 — — — — — — 27 0.120 2.15 0.21 — — — — — — 28 0.090 2.88 0.10— — — — — — 29 0.090 0.95 0.21 — — — — — — 30 0.090 3.81 0.17 — — — — —— 31 0.091 2.01 0.12 — — — — — — 32 0.090 2.11 0.25 — — — — — — 33 0.1102.61 0.29 — — — — 0.14 — 34 0.060 2.33 0.12 — — — — — 0.06 35 0.130 1.920.26 — — — — 0.02 0.15 36 0.060 2.13 0.26 0.0005 — — — — — 37 0.120 2.290.35 — 0.0015 — — — — 38 0.080 2.82 0.23 0.0003 0.0032 — — — — 39 0.0802.24 0.11 0.0003 0.0009 0.0005 0.0005 0.15 0.20 40 0.099 2.33 0.19 — — —— — —

The produced molten steels were used to produce blooms by continuouscasting. Next, similarly to Example 1, a blooming process was performedto produce round billets having a diameter of 310 mm. The in-furnacetemperature (° C.) and holding time (minutes) in the bloom reheatingfurnace were as shown in Table 4.

TABLE 4 Steel Material Production Process Total Blooming ProcessIn-furnace Residence In-furnace Holding Temperature Time t TemperatureTime in In-furnace Residence T (° C.) in (min) in Furnace in Bloom BloomTemperature Time in Heating Heating Time in Reheating Reheating inPreheating Zone and Zone and Heating Test Furnace Furnace PreheatingZone Holding Holding Furnace No. (° C.) (min) Zone (° C.) (min) ZoneZone FA (min) ΔCr 1 1260 255 1080 190 1250 447 4157 637 0.05 2 1250 2411090 170 1250 484 4326 654 0.05 3 1250 238 1070 178 1275 400 3997 5780.05 4 1250 220 1120 180 1250 401 3937 581 0.05 5 1250 203 1150 157 1250405 3957 562 0.05 6 1250 235 1050 188 1250 446 4152 634 0.05 7 1260 2671130 170 1250 485 4330 655 0.05 8 1250 268 1060 176 1250 526 4509 7020.04 9 1260 386 1140 184 1250 514 4458 698 0.04 10 1250 254 1100 1871250 492 4361 679 0.05 11 1260 301 1060 189 1250 436 4106 625 0.06 121260 330 1140 170 1250 443 4138 613 0.05 13 1270 233 1070 164 1250 4754285 639 0.05 14 1250 251 1090 160 1250 510 4440 670 0.05 15 1250 2771080 172 1250 509 4436 681 0.04 16 1260 280 1060 180 1250 489 4348 6690.05 17 1260 312 1110 180 1250 490 4352 670 0.06 18 1260 326 1130 1911250 544 4586 735 0.04 19 1260 299 1070 185 1250 443 4138 628 0.05 201270 204 1070 170 1250 423 4044 593 0.06 21 1250 256 1120 184 1250 4394120 623 0.05 22 1250 245 1110 174 1250 520 4484 694 0.04 23 1260 2121070 166 1250 428 4068 594 0.05 24 1270 294 1080 169 1225 474 4210 6430.04 25 1250 344 1110 200 1250 531 4531 731 0.04 26 1250 360 1060 1851250 488 4343 673 0.09 27 1260 221 1110 175 1250 544 4586 719 0.06 281250 238 1100 182 1250 447 4157 629 0.11 29 1270 262 1130 191 1250 4844326 675 0.05 30 1260 300 1120 175 1250 422 4039 597 0.09 31 1260 2571090 160 1250 341 3631 501 0.07 32 1250 226 1090 170 1250 288 3337 4580.06 33 1270 300 1070 176 1250 317 3501 493 0.07 34 1270 208 1150 1651250 304 3428 469 0.05 35 1260 240 1140 168 1250 313 3479 481 0.08 361250 354 1090 174 1250 326 3550 500 0.07 37 1250 288 1110 158 1250 3163495 474 0.05 38 1250 303 1120 158 1250 344 3647 502 0.07 39 1250 2901150 154 1250 328 3561 482 0.06 40 1260 266 1060 156 1225 403 3882 5590.06 Tempering Process Holding Yield Test Temperature Time (MPa) No. ΔMoΔCu ΔF (° C.) (min) Strength SSC Resistance 1 0.22 0.18 0.45 606 23 915P 2 0.26 0.12 0.43 580 25 938 P 3 0.23 0.19 0.47 602 40 881 P 4 0.230.17 0.45 605 39 885 P 5 0.26 0.11 0.42 592 40 902 P 6 0.28 0.14 0.47606 26 903 P 7 0.26 0.14 0.45 587 31 920 P 8 0.26 0.12 0.42 590 35 893 P9 0.24 0.13 0.41 599 20 919 P 10 0.27 0.11 0.43 599 34 901 P 11 0.110.27 0.44 610 22 890 P 12 0.22 0.19 0.46 605 25 916 P 13 0.20 0.17 0.42599 22 938 P 14 0.29 0.13 0.47 585 27 941 P 15 0.29 0.13 0.46 580 24 946P 16 0.26 0.12 0.43 591 29 912 P 17 0.29 0.08 0.43 591 36 937 P 18 0.200.18 0.42 588 22 939 P 19 0.20 0.20 0.45 605 29 914 P 20 0.34 0.08 0.48609 36 890 P 21 0.23 0.19 0.47 610 35 904 P 22 0.29 0.10 0.43 586 21 904P 23 0.25 0.14 0.44 607 43 883 P 24 0.24 0.15 0.43 610 43 873 P 25 0.260.17 0.47 605 31 874 F 26 0.34 0.26 0.69 589 43 978 F 27 0.19 0.19 0.44604 38 891 F 28 0.31 0.32 0.74 588 23 963 F 29 0.30 0.08 0.43 590 40 914F 30 0.24 0.45 0.78 606 25 988 F 31 0.28 0.23 0.58 598 26 927 F 32 0.250.22 0.53 590 43 907 F 33 0.21 0.25 0.53 600 32 915 F 34 0.26 0.22 0.53590 41 909 F 35 0.32 0.14 0.54 585 32 938 F 36 0.35 0.17 0.59 585 22 950F 37 0.25 0.22 0.52 610 26 881 F 38 0.29 0.21 0.57 600 34 880 F 39 0.250.21 0.52 593 22 905 F 40 0.32 0.17 0.55 595 39 902 F

Next, similarly to Example 1, the round billet of each test number wassubjected to a steel material production process. In the steel materialheating process, the in-furnace temperature (° C.) in the preheatingzone, the residence time (minutes) in the preheating zone, thein-furnace temperature T (° C.) in the heating zone and the holdingzone, and the total residence time t (minutes) in the heating zone andthe holding zone were as shown in Table 4. Further,FA=(t/60)^(0.5)×(T+273) was as shown in Table 4.

Each heated round billet was subjected to hot working under the sameconditions as in Example 1 to thereby produce a hollow shell for eachtest number. In addition, each produced hollow shell was subjected to aheat treatment process (quenching process and tempering process). In thequenching process, the quenching temperature was set to 910° C., and theholding time at the quenching temperature was set to 15 minutes. In thetempering process, the tempering temperature (° C.) was set as shown inTable 4, and the holding time (minutes) at the tempering temperature wasset as shown in Table 4. The yield strength was adjusted to 125 ksi ormore (862 MPa or more) by the heat treatment process. Martensiticstainless steel materials (seamless steel pipes) were produced by theabove production process.

[Evaluation Tests]

The seamless steel pipe of each test number was subjected to thefollowing evaluation tests by the same methods as the methods employedin Example 1.

-   -   (1) Microstructure observation test    -   (2) Cr concentration, Mo concentration, and Cu concentration        measurement test    -   (3) Tensile test    -   (4) SSC resistance evaluation test

The result of the microstructure observation test showed that, in eachtest number, the volume ratio of martensite was 80.0% or more. Theresults for degree of Cr segregation ΔCr, degree of Mo segregation ΔMo,degree of Cu segregation ΔCu, ΔF, yield strength, and SSC resistanceevaluation obtained in the evaluation tests of (2) to (4) mentionedabove are shown in Table 4.

[Evaluation results]

Referring to Table 4, in Test Numbers 1 to 24, the content of eachelement in the chemical composition was within the range of the presentembodiment. In addition, in the heating process, the in-furnacetemperature and residence time in the preheating zone were appropriate,the in-furnace temperature Tin the heating zone and the holding zone was1225 to 1275° C., and FA was 3900 or more. Therefore, the total degreeof segregation ΔF was 0.50 or less, and the Cr concentrationdistribution, the Mo concentration distribution, and the Cuconcentration distribution in a microscopic segregation region in thesteel material were sufficiently uniform. As a result, the yieldstrength was 125 ksi grade or more (862 MPa or more), and excellent SSCresistance was obtained.

On the other hand, in Test Number 25 the content of Cr was too low.Therefore, the SSC resistance was low.

In Test Number 26 the content of Cr was too high. Therefore, the totaldegree of segregation ΔF was more than 0.50. As a result, the SSCresistance was low.

In Test Number 27 the content of Mo was too low. Therefore, the SSCresistance was low.

In Test Number 28 the content of Mo was too high. Therefore, the totaldegree of segregation ΔF was more than 0.50. As a result, the SSCresistance was low.

In Test Number 29 the content of Cu was too low. Therefore, the SSCresistance was low.

In Test Number 30, the content of Cu was too high. Therefore, the totaldegree of segregation ΔF was more than 0.50. As a result, the SSCresistance was low.

In Test Numbers 31 to 40, although the content of each element in thechemical composition was within the range of the present embodiment, FAwas less than 3900 and Formula (A) was not satisfied. Therefore, thetotal degree of segregation ΔF in these test numbers was more than 0.50.As a result, in these test numbers the SSC resistance was low.

An embodiment of the present disclosure has been described above.However, the foregoing embodiment is merely an example for implementingthe present disclosure. Accordingly, the present disclosure is notlimited to the above embodiment, and the above embodiment can beappropriately modified and implemented within a range which does notdeviate from the gist of the present disclosure.

REFERENCE SIGNS LIST

-   -   10 Heating furnace    -   100 Billet    -   SE Segregation region    -   Z1 Preheating zone    -   Z2 Heating zone    -   Z3 Holding zone

1. A martensitic stainless steel material that is a seamless steel pipeor a round steel bar, having a chemical composition consisting of, inmass %: C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P:0.030% or less, S: 0.0050% or less, Ni: 5.00 to 7.00%, Cr: 10.00 to14.00%, Mo: 1.50 to 3.00%, Al: 0.005 to 0.050%, V: 0.01 to 0.30%, N:0.0030 to 0.0500%, Ti: 0.020 to 0.150%, Cu: more than 1.00 to 3.50%, Co:0.50% or less, B: 0 to 0.0050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, rareearth metal (REM): 0 to 0.0050%, Nb: 0 to 0.15%, and W: 0 to 0.20%, withthe balance being Fe and impurities, wherein: a yield strength is 758MPa or more; in a case where the martensitic stainless steel material isthe seamless steel Pipe, when, in a cross section including a rollingdirection and a wall thickness direction of the seamless steel pipe, anarbitrary two points at positions at a depth of 2 mm from an innersurface are defined as two center points P1, and two line segments of1000 μm extending in the wall thickness direction with each center pointP1 as a center are defined as two line segments LS, energy dispersiveX-ray spectroscopy is performed at measurement positions at a pitch of 1μm on each line segment LS, and a Cr concentration, a Mo concentration,and a Cu concentration at each measurement position are determined; in acase where the martensitic stainless steel material is the round steelbar, when, in a cross section including a rolling direction and a radialdirection of the round steel bar, an arbitrary two points on a centralaxis of the round steel bar are defined as two center points P1, and twoline segments of 1000 μm extending in the radial direction with eachcenter point P1 as a center are defined as two line segments LS, energydispersive X-ray spectroscopy is performed at measurement positions at apitch of 1 μm on each line segment LS, and a Cr concentration, a Moconcentration, and a Cu concentration at each measurement position aredetermined; and when: an average value of all of the Cr concentrationsdetermined at all of the measurement positions on the two line segmentsLS is defined as [Cr]_(ave), a sample standard deviation of all of theCr concentrations determined at all of the measurement positions on thetwo line segments LS is defined as σ_(Cr), among all of the Crconcentrations determined at all of the measurement positions on the twoline segments LS, an average value of the Cr concentrations includedwithin a range of [Cr]_(ave)±3σ_(Cr) is defined as [Cr*]_(ave), amongall of the Cr concentrations determined at all of the measurementpositions on the two line segments LS, a maximum value of the Crconcentrations included within a range of [Cr]_(ave)±3σ_(Cr) is definedas [Cr*]_(max), among all of the Cr concentrations determined at all ofthe measurement positions on the two line segments LS, a minimum valueof the Cr concentrations included within a range of [Cr]_(ave)±3σ_(Cr)is defined as [Cr*]_(min), an average value of all of the Moconcentrations determined at all of the measurement positions on the twoline segments LS is defined as [Mo]_(ave), a sample standard deviationof all of the Mo concentrations determined at all of the measurementpositions on the two line segments LS is defined as σ_(Mo), among all ofthe Mo concentrations determined at all of the measurement positions onthe two line segments LS, an average value of the Mo concentrationsincluded within a range of [Mo]_(ave)±3σ_(Mo) is defined as [Mo*]_(ave),among all of the Mo concentrations determined at all of the measurementpositions on the two line segments LS, a maximum value of the Moconcentrations included within a range of [Mo]_(ave)±3σ_(Mo) is definedas [Mo*]_(max), among all of the Mo concentrations determined at all ofthe measurement positions on the two line segments LS, a minimum valueof the Mo concentrations included within a range of [Mo]_(ave)±3σ_(Mo)is defined as [Mo*]_(min), an average value of all of the Cuconcentrations determined at all of the measurement positions on the twoline segments LS is defined as [Cu]_(ave), a sample standard deviationof all of the Cu concentrations determined at all of the measurementpositions on the two line segments LS is defined as σ_(Cu), among all ofthe Cu concentrations determined at all of the measurement positions onthe two line segments LS, an average value of the Cu concentrationsincluded within a range of [Cu]_(ave)±3σ_(Cu) is defined as [Cu*]_(ave),among all of the Cu concentrations determined at all of the measurementpositions on the two line segments LS, a maximum value of the Cuconcentrations included within a range of [Cu]_(ave)±3σ_(Cu) is definedas [Cu*]_(max), and among all of the Cu concentrations determined at allof the measurement positions on the two line segments LS, a minimumvalue of the Cu concentrations included within a range of[Cu]_(ave)±3σ_(Cu) is defined as [Cu*]_(min), a degree of Cr segregationΔCr defined by Formula (1), a degree of Mo segregation ΔMo defined byFormula (2), and a degree of Cu segregation ΔCu defined by Formula (3)satisfy Formula (4):ΔCr=([Cr*]_(max)−[Cr*]_(min))/[Cr*]_(ave)  (1)ΔMo=([Mo*]_(max)−[Mo*]_(min))/[Mo*]_(ave)  (2)ΔCu=([Cu*]_(max)−[Cu*]_(min))/[Cu*]_(ave)  (3)ΔCr+ΔMo+ΔCu≤A  (4) where, in a case where the yield strength is 758 toless than 862 MPa, A in Formula (4) is 0.70, and in a case where theyield strength is 862 MPa or more, A in Formula (4) is 0.50.
 2. Themartensitic stainless steel material according to claim 1, wherein thechemical composition contains one or more elements selected from thegroup consisting of: B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, Mg:0.0001 to 0.0050%, rare earth metal (REM): 0.0001 to 0.0050%, Nb: 0.01to 0.15%, and W: 0.01 to 0.20%.